Plasma Physics and Controlled Fusion

TOPICAL REVIEW Nuclear diagnostics for Inertial Confinement Fusion (ICF) plasmas

To cite this article: J A Frenje 2020 Plasma Phys. Control. Fusion 62 023001

View the article online for updates and enhancements.

This content was downloaded from IP address 18.21.213.6 on 07/01/2020 at 20:53 Plasma Physics and Controlled Fusion

Plasma Phys. Control. Fusion 62 (2020) 023001 (44pp) https://doi.org/10.1088/1361-6587/ab5137

Topical Review Nuclear diagnostics for Inertial Confinement Fusion (ICF) plasmas

J A Frenje

Massachusetts Institute of Technology, Cambridge, MA 02139, United States of America

E-mail: [email protected]

Received 12 December 2017, revised 13 September 2019 Accepted for publication 25 October 2019 Published 7 January 2020

Abstract The field of nuclear diagnostics for Inertial Confinement Fusion (ICF) is broadly reviewed from its beginning in the seventies to present day. During this time, the sophistication of the ICF facilities and the suite of nuclear diagnostics have substantially evolved, generally a consequence of the efforts and experience gained on previous facilities. As the fusion yields have increased several orders of magnitude during these years, the quality of the nuclear-fusion-product measurements has improved significantly, facilitating an increased level of understanding about the physics governing the nuclear phase of an ICF implosion. The field of ICF has now entered an era where the fusion yields are high enough for nuclear measurements to provide spatial, temporal and spectral information, which have proven indispensable to understanding the performance of an ICF implosion. At the same time, the requirements on the nuclear diagnostics have also become more stringent. To put these measurements into context, this review starts by providing some historical remarks about the field of ICF and the role of nuclear diagnostics, followed by a brief overview of the basic physics that characterize the nuclear phase and performance of an ICF implosion. A technical discussion is subsequently presented of the neutron, gamma-ray, charged-particle and radiochemistry diagnostics that are, or have been, routinely used in the field of ICF. This discussion is followed by an elaboration of the current view of the next-generation nuclear diagnostics. Since the seventies, the overall progress made in the areas of nuclear diagnostics and scientific understanding of an ICF implosion has been enormous, and with the implementation of new high-fusion-yield facilities world-wide, the next- generation nuclear diagnostics will play an even more important role for decades to come.

Keywords: nuclear diagnostics, neutron, gamma-ray, charged particle, plasma, Inertial Confinement Fusion, ICF (Some figures may appear in colour only in the online journal)

1. Introduction filled with deuterium–tritium (DT) fuel, positioned inside a small cylindrical hohlraum, could be imploded by x-rays 1.1. Inertial Confinement Fusion (ICF)—historical remarks produced in the hohlraum [1, 2]. However, at the time it was The history of ICF can be traced back to a meeting organized not clear how to heat the hohlraum and generate x-rays and by Edward Teller in 1957, which focused on the use of drive the capsule. With the introduction of the laser in 1960, it hydrogen bombs for peaceful purposes. One of the outcomes became clear that the laser represented a suitable driver for of that meeting was the initiation of John Nuckolls work on ICF applications. As a result, a large body of theoretical and significantly scaling down the secondary (the fusion part of a computational work was conducted in the United States (US), hydrogen bomb), while still generating energy gain. This Europe, Japan and the Union of Soviet Socialist Republics early work pointed to the idea that a small spherical capsule (USSR) in the 60s and early 70s, which laid the foundation

0741-3335/20/023001+44$33.00 1 © 2020 IOP Publishing Ltd Printed in the UK Plasma Phys. Control. Fusion 62 (2020) 023001 Topical Review for laser-driven ICF, both in direct-drive mode (where lasers ∼1013 [19]. At NOVA, the first fully integrated indirect-drive directly illuminate the capsule), and indirect-drive mode experiments were conducted in 1985 [20], and in 1990, (where lasers illuminate the inside of a hohlraum for pro- experiments with the upgraded NOVA laser symmetrically duction of x-rays that drive the capsule). Other types of driver compressed DT implosions about 100× in a 300 eV hohlraum concepts were also investigated such as electrons, heavy ions with little preheat. The enhanced NOVA capability also and magnetic fields [3, 4]. This era culminated in 1972 with produced neutron yields up to ∼1013, which were made the first unclassified ICF paper ‘Laser Compression of Matter possible by increasing the energy output per beam and by to Super-High Densities’ published by Nuckolls et al in substantially improving the suite of diagnostics. In parallel, a Nature [5], which formulates the foundation for the direct- series of implosion experiments with the OMEGA-24 system drive ICF scheme where 100 kJ–1 MJ lasers are used to demonstrated 100–200× liquid-DT density in 1988 [21], implode a millimeter-sized spherical capsule, filled with DT which was facilitated in part by implementing and using an gas, to densities and temperatures high enough for significant extensive array of diagnostics. In 1996, OMEGA-24 was thermonuclear fusion, ignition and energy gain to occur. The upgraded to OMEGA-60 [22] to achieve better uniformity scheme and the many challenges presented in this seminal and to lay the foundation for validating the direct-drive paper are still relevant today. approach for achieving ignition. After Nuckolls’ paper was published, several large-scaled The findings from the experiments in the 80s and early (hundreds of joule) laser facilities were built or used to 90s were reviewed by the National Academy of Science that explore the spherical implosion concept. In Japan, the Gekko- issued the Koonin Report [23], which stated that most mile- II laser was used to conduct spherical implosion experiments stones in the efforts in achieving the required implosion in 1973 [6, 7]. In 1974, the KMS Fusion in the US used a conditions had been met to initiate a large-scaled ignition two-beam laser system, capable of delivering up to 200 J in program. In that report, it was concluded that the predicted 100 ps pulse, to irradiate a set of DT gas-filled capsules, fuel densities were achieved, while neutron yields were orders which generated thermonuclear conditions and neutron yields of magnitude lower than predicted, indicating that the for- up to ∼107 [8]. At Lawrence Livermore National Laboratory mation of the hot spot and resulting ion temperature were (LLNL), similar experiments were conducted in 1974 with the impeded mainly by Rayleigh–Taylor (RT) instabilities. It was single-beam JANUS laser, and in 1975 neutron yields in also concluded that the required conditions for ignition could excess of 108 were generated [9]. By early 1976, the upgraded not be achieved with the facilities at hand, which initiated the 2-beam ARGUS laser generated ion temperatures of ∼10 keV effort of building the MJ-class lasers: the National Ignition and neutron yields up to ∼1010, which represented a big step Facility (NIF) [24] at LLNL, and the Laser Mega Joule (LMJ) forward in terms of performance. At about the same time, the [25] in France; which are primarily designed for indirect-drive six laser-beam ZETA system was built at the Laboratory for experiments. At the NIF, the first light was obtained in 2003, Laser Energetics (LLE) at the University of Rochester, where while the full operation of the system started in 2009, with the a series of symmetrical implosion experiments utilizing gas- first phase being the National Ignition Campaign that pro- filled, thin-glass capsules was carried out. These experiments duced highest fuel areal density (ρR) to date (∼1.6 g cm2) generated impressive results in terms of implosion symmetry [26]. This campaign was followed by several campaigns that and neutron yields up to 1.5×109 [10]. involved more stable implosions, which led to a hot-spot The downside with these first experiments was that they pressures in excess of 350 Gbar and record neutron yields of were performed with thin-glass capsules that could not gen- ∼2×1016 [27, 28]. The LMJ system started partial operation erate the required densities for ignition to occur. The program in 2014. In parallel, a cryogenic DT campaign at OMEGA-60 at LLNL was therefore redirected toward implosions based on was initiated in 2006 to validate the direct-drive approach for isentropic compression required for high convergence and achieving ignition. In the subsequent years, several important high fuel densities. In 1978, fuel densities of order 10× liquid milestones were achieved, culminating in a ρR of density were achieved in experiments with the 2 kJ ARGUS ∼300 mg cm−2 [29], hot-spot pressure of 56 Gbar and neu- laser, and about a year later fuel compression of about 100× tron yields in excess of 1014 [30]. was achieved with the 10 kJ SHIVA laser [11]. The ion The Chinese ICF program continued to advance the SG- temperature in these experiments was low (∼0.5 keV) to series. In 2001 the SG-II facility was operated at 6 kJ, and in maximize fuel density but to provide sufficient thermonuclear 2014 the SG-II upgrade operated at 24 kJ [31]. The 48-beam, yields for diagnostics purposes [11]. Several experiments at 180 kJ SG-III laser facility was commissioned in 2015 and SHIVA also explored the indirect-drive concept using subsequently started operation [32]. In addition, the MJ-class hohlraums. SG-IV laser represents the next step in China’s ICF program, The results from these first experiments were very and is being designed to explore ignition and burn [33]. This encouraging and led to the development of several new, larger facility is projected to be in operation within the next decade. ICF facilities around the world, such as NOVA in the US In Russia, the 192-beam, 2.8 MJ UFL-2M laser is currently [12], Gekko-XII in Japan [13], Phebus in France [14], being built at the Russian Federal Nuclear Center in Sarov ISKRA-4 (followed by ISKRA-5) in the USSR [15, 16], SG-I [34]. This system is a 2ω laser system for direct-drive in China [17] and OMEGA-24 in the US [18]. Hot-spot experiments. experiments at Gekko-XII generated impressive results, The fast-ignition approach in which the fuel compression including ion temperatures of ∼10 keV and neutron yields of is decoupled from the heating of the hot spot [35], has been

2 Plasma Phys. Control. Fusion 62 (2020) 023001 Topical Review explored in parallel with the standard hot-spot approach. Fast heats and radially expands outward. As the outer part of the ignition experiments have been conducted at Gekko-XII, ablator expands, the remaining inner part is forced inward to culminating in neutron yields up to 3.5×107 [36, 37]. The conserve momentum. This process is generally done quasi- facilities that are being implemented and have been proposed isentropically [42, 43], and at peak compression the fuel is to pursue fast ignition are Fast Ignition Research Experiment nearly isobaric and typically represented by two regions: a hot at Osaka University [38], and High Power laser Energy central plasma (called hot spot), containing a small fraction of Research facility in the United Kingdom (UK) [39], respec- the fuel, and a denser (and colder) fuel-shell surrounding the tively. If fast-ignition approach is successfully implemented hot spot. The fuel-shell has several functions: first, it com- and ignition is achieved, it would in principle lower the presses and heats the hot spot by acting as a piston, secondly, amount of energy required to be coupled to the capsule by an it provides inertial confinement, and thirdly, it traps the dt- order of magnitude. alphas for self-heating. At peak compression, the fusion Another promising approach that has recently emerged in process is initiated by several carefully tailored shocks that the field of ICF is the Magnetized Liner Inertial Fusion converge at the center of the implosion, significantly raising (MagLIF) method [40]. This method, which was developed at the ion temperature and density of the hot spot. If the temp- Sandia National Laboratory, uses imploding magnetic fields erature and density are high enough, hot-spot ignition occurs. of a cylindrical beryllium liner (Z-pinch) combined with a In addition, if the ρR of the surrounding fuel-shell is sufficient heater laser beam. The MagLIF concept is today mainly to stop majority of the dt-alphas and the fuel is kept together operating with pure D2 fuel, and has to date generated up to for long enough time, a thermonuclear burn wave will prop- ∼1013 neutrons [41]. The long-term plan for the MagLIF agate through the main fuel-shell. The energy coupled to the approach is to gradually switch over to DT fuel operation. hot spot is small (a few kJ), but if hot-spot ignition is followed by burn of the main fuel, the energy output is amplified 1.2. Role of nuclear diagnostics significantly. As discussed in the literature [44], a simple form that In retrospect, each decade has displayed the development of represents the ignition condition for a steady-state plasma can ICF facilities and nuclear diagnostics that were orders of be derived by balancing the alpha-power density with the magnitude more capable than those used during the preceding power-loss density mainly due to heat loss caused by fuel- decade. This is generally a consequence of the fact that the shell expansion and thermal conduction3. Often referred to a efforts, lessons learned, and experience gained on previous Lawson-type criterion, the ideal ignition condition is facilities enabled more sophisticated diagnostics on the next- expressed as generation facilities. Higher fusion yields also facilitated higher-quality measurements, which facilitated an increased n22P áñsv 3 P eseáñ=v > ,2.1() level of understanding about the physics governing the aa2 416Ti 2 tE nuclear phase of an ICF implosion. This in turn provided essential guidance of the ICF programs, resulting in where εα is the kinetic energy of the DT-alpha, n is the total improvements in the fusion yield. The field of ICF has now ion-number density, 〈σv〉 is the fusion reactivity for a thermal entered an era where the fusion yields are high enough for plasma, P is the hot-spot pressure, Ti is the hot-spot ion τ fi 4 unprecedented nuclear measurements with spatial, temporal temperature, and E is the energy-con nement time . Here, it or spectral information, which are indispensable in under- is assumed that the pressure profile is constant in the hot spot = ( ) standing the performance of an ICF implosion. With the and thus can be expressed as P 2nTi. Equation 2.1 can be implementation of new high-fusion-yield facilities world- rewritten as wide, the next-generation nuclear diagnostics will play an 2 24Ti even more important role for decades to come. PtE > .2.2() esa áñv As shown by equation (2.2), the Lawson criterion depends on 2. Brief overview of the physics governing the 2 2 Ti . In the case of an ICF implosion, the Ti /〈σv〉 is mini- nuclear phase of an ICF implosion mized for a Ti of ∼13 keV for the dt reaction [45]. However, to achieve this temperature using the spherical-implosion The concept of an ICF implosion involves a spherical capsule concept, the implosion velocity must be ∼700 km s−1, which ( ) ( ) made of either plastic CH , high-density carbon HDC ,or is unrealistically high because the implosion is hydro- ( ) fi 1 beryllium Be that is lled with milligrams of DT fuel in the dynamically unstable and severely breaks up in-flight. A form of gas at vapor pressure at the center surrounded by a lower implosion velocity of ∼400 km s−1, which generates T [ ]2 i cryogenic ice-layer on the inside of the capsule 42 . In the of ∼5 keV, is optimal for a stable implosion. With these case of laser-driven ICF, the energy from the driver is implosion parameters, the ideal ignition condition PτE is delivered to the outer part of the capsule (or ablator), which 3 When the density in the surrounding fuel-shell is high, most of the 1 D and T represent a deuterium and tritium atom, respectively. This should radiation is absorbed in the fuel-shell and the energy is recycled back into the be contrasted to d and t that represent a deuteron and triton, respectively. hot spot by increasing the ablation of the inner surface of the fuel-shell. 2 4 In the case of MagLIF, the implosion involves a cylindrical Be liner filled τE represents the characteristic plasma-energy relaxation time due to with pure D2 gas in most experiments. electron heat conduction.

3 Plasma Phys. Control. Fusion 62 (2020) 023001 Topical Review

∼20 atm s (and not PτE,min∼8 atm s, which is required for a As discussed in the next sections, this can be done through Ti of ∼13 keV). spectral, spatial and temporal measurements of the nuclear To assess the performance of an ICF implosion and how fusion products emitted from an ICF implosion. it compares to the ideal ignition condition, the experimental Pτ is often normalized to the ideal ignition condition PτE,min. This ratio is called the ignition parameter (χ) [45]. During the 3. Nuclear fusion products from an ICF implosion deceleration phase of the implosion, the kinetic energy of the fuel-shell is converted into hot-spot thermal energy, which In an ICF implosion with DT fuel, a certain set of reactions 2 3 means that the hot-spot pressure scales as P∼mv /R , where take place. In surrogate implosions with either pure D2 fuel or m∼ρR(R2) represents the fuel-shell mass, R represents the D3He fuel, another set of reactions take place. In this review fuel radius, and v∼R/τ is the implosion velocity (where τ paper, we discuss the primary, secondary and tertiary reac- represents the implosion time scale). Using these relations, χ tions, in these fuels, which are used routinely to diagnose an can be expressed as ICF implosion5. A detailed discussion about the information carried by the fusion products is made in section 4. Pt c =~rRvT··i,2.3 ( ) PtE,min 3.1. Primary where ρR is the burn-averaged areal density of the fuel-shell, The primary reactions and reaction products that are used and Ti is the burn-averaged ion temperature. Here, it is routinely for diagnostics purposes are: 〈σ 〉 3 assumed that v is approximately proportional to Ti in the dt+4He()()() 3.56 MeV + n 14.03 MeV , 100% 3.1 range of 3.5–7.0 keV. On the basis of analytical and numer- 55 -5 ical modeling, Betti et al showed that the implosion velocity v +He* Heg0() 16.75 MeV , 4 ´ 10 () 3.2 ( ) 0.8/ρ 0.2 [ ] 55 -4 in equation 2.3 can be expressed as Ti R 45 , which +~He** Heg1() 13 MeV , 10 () 3.3 leads to dd+3He()()() 0.82 MeV + n 2.45 MeV , 50% 3.4 1.6 ⎛ T ⎞ 3 cr» R0.8⎜⎟i ,2.4() +H()()() 1.01 MeVp 3.02 MeV , 50% 3.5 ⎝ 4.7 ⎠ 44 -7 +He* Heg0() 23.8 MeV , 10 () 3.6 and tt+4He()()() < 3.8 MeV + 2 n < 9.4 MeV , 85% 3.7 PRTtr> 8,·( · )0.8 ( 2.5 ) i +5He()() 1.7 MeVn 8.7 MeV , 10% () 3.8 where Pτ is given in atm s, ρR is given in g cm−2 and T is i +5He*()() 1.4 MeVn 7.0 MeV , 5% () 3.9 given in keV. It is important to note that these expressions were derived in the limit of one dimension (1D). In three dp+34He He()() 3.73 MeV + 14.63 MeV , 100% dimensions (3D), the effect of asymmetries [46] and RT- ()3.10 induced mix of hot and cold fuel must also be considered ++55He* Heg () 16.7 MeV , 5 ´ 10-5 () 3.11 [47, 48], as they can impact the proper formation of the hot 0 34 spot and reduce Ti, burn volume and Yn due to reduced td+He He()()() 4.77 MeV + 9.55 MeV . 41% 3.12 pressure. To account for the 3D effects, Betti et al showed Here, the kinetic energies of the fusion products for a zero- ( ) ( ) numerically that equations 2.4 , 2.5 should be reformulated temperature plasma are shown in the parentheses. The as strength of the different reaction branches is also indicated. ⎛ ⎞1.6⎛ ⎞0.4 Among these reactions, the d+t (dt) reaction is the most TYin cr= R0.8⎜⎟⎜ ⎟ ,2.6()viable one from an energy gain point of view due to its ⎝ 4.3 ⎠ ⎝Y ⎠ 1D highest cross-section (about two orders of magnitude higher and than the cross sections for the other reactions) at temperatures 0.4 that can be readily achieved in an ICF implosion. In the dt ⎛ Y ⎞ PRTtr> 8,2.7·( · )0.8⎜ n ⎟ ( ) case, 80% of the kinetic energy is carried by the neutron, i ⎝ ⎠ Y1D while the remaining kinetic energy is carried by the alpha where Yn and Y1D are the measured and 1D-simulated neutron particle. yields, respectively. The assumption made here is that 3D effects fully explain the discrepancy between simulations 3.2. Secondary and experiments. It should also be noted that from In an ICF implosion, a large fraction of the primary-reaction equations (2.4)–(2.7) it is clear that the DT fuel must to be products interact (or react) with the ions in the fuel and spherically assembled to maximize ρR and T , while at the i ablator (or liner). In the case of the DT fuel, the dt neutrons same time keep the fuel assembled for long enough time for a substantial number of fusion reactions to take place. To 5 In some experiments, a dopant of a particular element is introduced into the evaluate the performance of an ICF implosion, during the fi ρ fuel or ablator, targeting a speci c reaction that can be measured by a certain nuclear phase, it is also clear that Yn, Ti, R, burn duration, set of nuclear diagnostics probing certain physics. These particular cases are burn profiles, and their 3D asymmetries must be diagnosed. discussed in some of the diagnostics sections.

4 Plasma Phys. Control. Fusion 62 (2020) 023001 Topical Review interact (or react) with the ions in the fuel and ablator (or involves primary dd neutrons elastically scattering off the Be liner) as described by liner (in a MagLIF experiment), as described by the reaction ndd()(14.03 MeV+¢< 12.47 MeV ) n()2.45 MeV+<99 Be Be ( 0.88 MeV ) +¢n (–1.56 14.03 MeV ) , () 3.13 +¢n (–1.57 2.45 MeV ) . () 3.19 ntt()(14.03 MeV+¢< 10.52 MeV ) Other secondary-reaction processes that are used to diagnose +¢n (–3.51 14.03 MeV ) , () 3.14 an implosion are initiated by the 1.01 MeV tritons and 0.82 MeV 3He-ions generated by the dd reactions (reactions npp()(14.03 MeV+¢< 14.03 MeV ) (3.4) and (3.5)). A small fraction of these nuclear-fusion +¢tokamak plasmas. In the case of pure D2 fuel, there are several secondary- As fusion yields have increased about ten orders of magnitude reaction processes that take place in an implosion, but only a since the beginning of the experimental ICF program, the few are relevant to ICF as discussed in section 4. One process quality of the nuclear-fusion-product measurements has

5 Plasma Phys. Control. Fusion 62 (2020) 023001 Topical Review

Figure 3. (a) The differential cross sections for the nd and nt elastic scattering at a neutron energy of 14 MeV. (b) The differential cross sections for the np elastic scattering, and n12C elastic and inelastic scattering. The elastic n12C scattering generates neutrons in the range 10.05–14.03 MeV, and inelastic n12C scattering on the first, second and third excited state of 12C generates neutrons in the energy range 6.3–9.5 MeV, 3.7–6.2 MeV and 2.1–4.1 MeV, respectively. Reproduced courtesy of IAEA. Figure adapted from [50]. Copyright 2013 IAEA.

Figure 1. ICF neutron spectra simulated by the hydrodynamic code indispensable to understanding the performance of an ICF LASNEX for three dt implosions at the NIF. The details of the implosion. In the case of a high-density ICF implosion, only spectra are dictated by primary, secondary and tertiary neutrons, as ( discussed in the text. The red spectrum is for an ignited implosion, neutral reaction products such as neutrons and gamma-rays at much lower yields) readily escape. Measurements of the producing a neutron-burn averaged Ti of ∼40 keV and Yn of 6×1018. The blue and black spectra are produced by implosions directional emission of primary, down-scattered and second- ∼ fi that failed to ignite, generating a Ti of 5 keV and signi cantly ary neutrons are therefore arguably the most important probe lower Yn. The blue spectrum is originating from an implosion with a of the conditions in the hot spot and surrounding high-density ρR of 2.0 g cm−2 that failed due to electron-conduction issues, while the black spectrum is originating from an implosion with a ρR of shell of an ICF implosion, as these types of neutrons carry 1.0 g cm−2 that failed due to entropy issues. As illustrated by these information about absolute nuclear yield, ion temperature, ρ two spectra (which have been normalized to each other using Yn), the nuclear burn history, fuel and ablator R, and morphology of Failure-1 implosion produces a significantly higher down-scattered- the implosion. In addition, primary and secondary charged neutron component than the Failure-2 implosion. Reproduced particles emitted from a lower- to medium-ρR implosion carry courtesy of IAEA. Figure from [50]. Copyright 2013 IAEA. similar information, as well as information about magnetic and electric fields. This is discussed in detail in this section.

4.1. Nuclear yield

Nuclear yield (Yij) is an integral quantity that provides important information about the overall performance of an ICF implosion. It depends on several factors and is normally expressed as

nnij Yij = ∬ áñstvdVdrel ij ,4.1() 1 + dij

where δij is the Kronecker delta that is equal to 1(0) for reactant i equal(unequal) to reactant j, ni and nj are the ion- number density of the reactants i and j, respectively, 〈σvrel〉ij is the fusion reactivity, and nini〈σvrel〉ij is the reaction rate that is integrated over the burn volume (V ) and burn duration (τ). The fusion reactivity is expressed as

áñ=ssvfvfvvvdvdvrel ij ∬ i ()()i j jrelreli ( ) j,4.2 ( )

Figure 2. Ti-dependence on the reactivity ratio (a) dt/dd, (b) dt/tt, 3 3 (c) dd/d He, and (d) dt/t He. where fi(vi) and fj(vj) are normalized velocity distributions of the two reactants, vrel is the relative velocity between fi improved signi cantly, facilitating an increased level of the two reactants, and σ(vrel) is the fusion cross section. In understanding about the physics governing the nuclear phase calculations of 〈σvrel〉ij and Yij, it is routinely assumed that of an ICF implosion. As discussed in this section, the field of the velocity distributions are Maxwellian. In addition, ICF has now entered an era where the fusion yields are high equation (4.1) implies that information about Ti, burn volume, enough to provide high-fidelity nuclear data with spatial, and burn duration and their spatial and temporal variations is temporal and spectral information, which have proven critical for evaluating the implosion performance.

6 Plasma Phys. Control. Fusion 62 (2020) 023001 Topical Review

keV. Here, ΔED is dictated by the center-of-mass-velocity (VCM) distribution of the reactants along the spectrometer line of sight (LOS). E4 is relativistically expressed as [57, 60] ⎛ ⎞ x mm2 - 2 Em=-()x +⎜ + 3 4 ⎟K 44 2 ⎝ ()()mm12+ mm12+ ⎠

1 2 2 2 ++xqxVVCM cos - m ,() 4.5 2 CM 4

where m1 and m2 are the masses of reactants 1 and 2, Figure 4. Birth spectra of up-scattered deuterons, tritons and protons, respectively, K is the total kinetic energy of the reactants in generated by 14 MeV neutrons elastically scattering of these ions. The the CM system, θ is the angle between the VCM direction and shape of the spectra are dictated by the differential cross-sections for the fusion-product direction in the CM system, and elastic scattering. The effective cross sections for the high-energy 2 2 2 ξ=[(m1 – m2) +m4 –m3 ]/[2(m1+m2)]. The first term deuteron and triton peaks are 104 mb and 142 mb, respectively. in equation (4.5) represents the energy of the fusion product 4 for a zero-temperature plasma (this term is dictated by the Q 4.2. Ion temperature value of the reaction as shown by equations (3.1)–(3.12)), the second term represents the energy upshift of fusion product 4 As discussed in section 2, Ti is a central performance metric due to a finite-temperature plasma, and the third and fourth for an ICF implosion. The information about Ti is encoded in the nuclear-yield ratio between two primary reactions and terms represent the CM energy. For a thermal plasma in Doppler-broadened primary fusion-product spectrum. This which VCM and emission direction of the fusion products are ( ) complementary information, when used concurrently, is isotropic, the last term in equation 4.5 vanishes, i.e. invaluable to the efforts in understanding the nature of the ⎛ 2 2 ⎞ x mm3 - 4 reacting ions. áñ=-Em44()x +⎜ + ⎟áñK ⎝ ()()mm+ mm+ 2 ⎠ In the case of a thermal plasma, the local velocity dis- 12 12 tribution of the reacting ions and 〈σv 〉 are uniquely 1 2 rel ij +áñx VCM . described by a local ion temperature T . This means that 2 i ()4.6 information about Ti is encoded in the yield ratio between two fl nuclear reactions with different reactivity sensitivity to Ti As discussed in the next section, macroscopic ows modify ( changes. For an equimolar plasma (ni=nj), Ti can be the VCM distribution and affect the mean energy 1st expressed in terms of reactivities as moment), width (2nd moment) and shape (higher-order moments) of the primary fusion-product spectrum. Yii 1 áñsvrel ii Ti µ= .4.3() Y 2 áñsv ij rel ij 4.3. Macroscopic plasma flows This expression is derived under the assumption that the burn The field of ICF has recently come to the realization that the V τ volume and burn duration are the same for the two nuclear width of the primary fusion-product spectrum is an inade- reactions. As the differences in burn duration and burn profile quate representation of Ti [61]. The main reason for this is that of the reacting constituents result in only minor corrections to macroscopic flows in the fuel are often comparable to the T ( the inferred i burn durations are similar and the different thermal velocities of the reactants, indicating that the width of T ) reactions are dominated by the high- i region near the center , the primary fusion spectrum is in fact a representation of both T the uncertainty associated with yield-ratio-inferred i is typi- thermal and macroscopic-flow variances. When considering cally smaller than the uncertainties in the quantities common to both these processes, Murphy showed that the width (ΔE) of V τ both reactions such as burn volume and burn duration .This the primary fusion-product spectrum can be expressed as [62] has been discussed by Casey et al for the dt/dd and dt/tt ratios [55]. Figure 2 illustrates the Ti-dependence on the reactivity ⎡ kT ⎤ D=D+D=EEE2 22 mE áñ⎢ Bi + s2 ⎥ DK32 ln 244⎣ KE⎦ . ratio for several light-ion reactions. mm34+ [ ] [ ] As discussed by Brysk 56 , Ballabio et al 57 , Appelbe ()4.7 et al [58], and Munro [59], Ti is also encoded in the width 2 fl (ΔE ) of the Doppler broadened primary fusion-product Here, sKE is the macroscopic- ow variance and is given by D / spectrum. For a thermal plasma, the shape of the spectrum is 2EKE 3M, where EKE is the kinetic energy of the fuel and M to the first order represented by a Gaussian, and T is is the total mass of the fuel. Given that kBTi can be expressed i 〈 〉 / 〈 〉 expressed in terms of ΔED as as A ETH 2MNA, where A is the average mass number of the reactants, ETH is the fuel thermal energy, and NA is the -5 mm34+ 2 Ti »´9 10DED ,() keV , () 4.4 Avogadro’s constant, equation (4.7) can be rewritten as mE44áñ 32 ln 2mEáñ⎡ 1 áñA 2 ⎤ where m and m are the masses of fusion products 3 and 4, D=E2 44⎢ EE+ ⎥. 3 4 ⎣ TH KE⎦ M 3 ()mmN+ A 3 respectively, and 〈E4〉 is the mean kinetic energy of the fusion 34 product 4 (given in MeV), and the width ΔED is given in ()4.8

7 Plasma Phys. Control. Fusion 62 (2020) 023001 Topical Review For an equimolar DT plasma where 〈A〉 is equal to 2.5, fusion product (per unit area normal to its direction of equation (4.8) points to the fact that the relative impact of EKE motion), ρR is a measure of the probability for interactions on ΔE is four times more effective than ETH, and that EKE between a fusion product and target ions in the fuel and affects the width of the dt-neutron spectrum 1.25 times more ablator. Given that nuclear cross sections govern neutron than the dd-neutron spectrum.6 From an optimal implosion interactions and that Coulomb cross sections govern charged- performance point of view, ETH and EKE must be maximized particle interactions, a higher ρR value enhances the yield of and minimized, respectively. down-scattered neutrons and secondary fusion products, As discussed by Gatu Johnson et al [60],Munroet al [61], whereas it increases the average energy downshift of charged and Murphy et al [62],macroscopicflows not only affect ΔE, fusion products emitted from an ICF implosion. More spe- but also 〈E4〉.Whileafinite Ti always increases 〈E4〉,amac- cifically, information about ρR is carried by roscopic plasma flow (Vfl ) either ‘red shifts’ or ‘blue shifts’ ow 1. Yield ratio between down-scattered neutrons and the primary fusion-product spectrum, depending on the effec- primary neutrons. tive velocity vector relative to the spectrometer LOS. Under the 2. Yield ratio between up-scattered ions and primary assumption that V ≈Vfl and K=Q, which is generally CM ow neutrons. the case for an ICF implosion, the mean energy shift (δEfl ) ow 3. Yield ratio between 4.44 MeV gamma-rays from 12C(n, due to macroscopic plasma flows is given by nγ) reactions and primary neutrons. 1 2 2 2 4. Relative yield between un-scattered primary neutrons dxqxEEEVVflow =-=40 +flow cos - m , 2 flow 4 emitted in different directions. ()4.9 5. Energy loss of charged fusion products. 6. Yield ratio between secondary dt neutrons (or d3He =ξ – [ ] where E0 m4. For the dt reaction, Gatu Johnson et al 60 protons) and primary dd neutrons. showed that equation (4.9) canbeapproximatedby 1. Yield ratio between down-scattered neutrons and -6 2 dqEVVflow =´5.3 10flow + 0.54·()flow cos . 4.10 primary neutrons −1 A fraction of the produced primary neutrons Here, Vfl is given in km s and δEfl is given in keV. For a ow ow elastically scatter of the fuel and ablator ions, generat- typical ICF implosion, Vfl is small, which means that the first ow ing an energy continuum of down-scattered neutrons as term can be neglected. For bulk velocities approaching −1 described by expressions (3.13)–(3.19). The shape and 1000 km s , the two terms start to be comparable. magnitude of the down-scattered neutron spectrum is, to the first order, dictated by the differential cross section 4.4. Nuclear burn profile and volume for the elastic-scattering process, and in some cases by Information about the fuel assembly of an ICF implosion is, both elastic and inelastic scattering processes, which are in part, encoded in the nuclear burn profile and overall burn generally well known [60–65] (see figure 3). volume, as indicated by equation (4.1). This type of data, The ratio between down-scattered-neutron yield combined with other data, carries information about fuel-shell (Yn′), in a certain energy range, and primary-neutron mix, implosion asymmetries, radiation and heat transport, and yield (Yn) is typically called the down-scattered ratio hot-spot pressure P. (dsr) and is an important performance metric for an ICF implosion. To illustrate the dependence between the 4.5. Nuclear-burn history total ρR and dsr, in the simplest possible fashion, a 1D hot-spot model of uniform dt fuel surrounded by CH The nuclear burn history provides critical information about ablator is used, where all primary dt neutrons are shock convergence, shock reverberations, dynamics of the produced in the center of the implosion. In this case, the high-density fuel-shell during the deceleration phase, impact total ρR is to the first order related to the dsr by7 of alpha heating, and a multitude of failure mechanisms. ⎛ Having this information is critical for assessing the energy- E2 ()23a + mp rrRR=+ r R =⎜ dsr ()E fi τ dt CH ò ⎝ dt con nement time E and performance of an ICF implosion, as E1 asnd+ s nt shown by equations (2.1)–(2.7) and (4.1). ⎞ ()4.11 ()b + 12 mp + dsrCH ()EdE⎟ . 4.6. Areal density of fuel and ablator bsnp+ s n12C ⎠ Information about the fuel assembly of an ICF implosion is encoded in the fuel and ablator ρR. Understanding spatial ρR Here, mp is the proton mass, and σnd, σnt, σnp, and σn12C asymmetries (discussed in section 4.8) are also central in our are the effective nd, nt, np and n12C cross sections for efforts in understanding the implosion performance. generating down-scattered neutrons in a certain energy As ρR represents the number of target ions in the fuel and interval E1 – E2, respectively. For an equimolar dt fuel ablator that are ‘in the way’ of a primary, secondary or tertiary and CH ablator, α=nd/nt=1 and β=np/nc=1. As

6 The coefficient in front of the thermal term in equation (4.8) is 1/6 for the 7 The first term in equation (4.11) is derived under the assumption that dt-neutron spectrum and 5/24 for the dd-neutron spectrum. ρR=∫ρRdr=(αmd+mt) ∫nt(r)dr and Yn=(ασnd+σnt)Yn ∫nt(r)dr.

8 Plasma Phys. Control. Fusion 62 (2020) 023001 Topical Review

8 [σnd+σnt](10–12 MeV) and [σnp+σn12C](10–12 MeV) generated, and the yield ratio between one of these ions are 400 mb and 250 mb, respectively, equation (4.11) is and primary neutrons is governed by the same physics reduced to9 dictating the dsr value. In the case of up-scattered ρ -2 deuterons, Rdt can in its simplest form be expressed in rrRR=+dt r R CH »21dsr dt + 87dsr CH .()() g cm 4.12 terms of (Ynd/Yn) as

This expression indicates that the down-scattered ()23a + mp Ynd rRdt = ,4.13() neutrons in the energy range 10–12 MeV primarily asnd Yn probe ρRdt and are relatively insensitive to the ρRCH due to different interaction probabilities per unit ρR. where σ is the effective cross section for generating According to radiation hydrodynamic simulations, nd up-scattered deuterons in a certain energy interval. ρR is also typically 10%–20% of the ρR in an CH dt Similar expressions exist for up-scattered tritons and implosion at the NIF [50], indicating that thee second protons from the fuel and CH ablator, respectively. For term in equation (4.12) can often be omitted. However, − low-ρR cases (<50 mg cm 2), the high-energy peak of as the total ρR dictates the confinement time τ of the hot dt the up-scattered deuteron and triton spectra are spot, information about ρR is also important. If a CH effectively used to infer ρR . At higher ρR values, different ablator is used, the coefficient in front of the dt dt most of these ions are ranged out, significantly second term is different. In addition, as ρR increases, distorting the spectrum, which prevents the use of the equations (4.11), (4.12) gradually become invalid high-energy peak [67]. Up-scattered protons from the because multiple scattering becomes increasingly ρ 10 CH ablator provide information about the RCH up to important . − ∼250 mg cm 2. Since the differential cross section and Although the dsr value provides information about spectrum of up-scattered protons are practically flat, the ρR , it does not provide sufficient information for dt yield per unit energy (Y ) can be used for a ρR building a complete picture of the assembled fuel. np CH determination as described by Additional information about the birth location of the primary neutrons and scattering locations generating the ()b + 12 mYp np rRCH = ,4.14() down-scattered neutrons is necessary. This information bsnp Yn can in principle be accessed because the scattering angle between the incoming primary and outgoing down- σ scattered neutron uniquely defines the energy of the where np is the effective cross section for generating up-scattered protons in a certain energy range. down-scattered neutron as described by En′=En 2 2 3. Yield ratio between 4.44 MeV gamma-rays and primary [1–4ACos θ/(A+1) ][66], where En is the energy of the primary neutron, A is the mass number of target neutrons ion, and θ is the scattering angle between the incoming A small fraction of the produced primary neutrons 12 primary neutron and outgoing recoil ion. This expres- inelastically scatter of the CnucleiinaCHorHDC ( ( )) sion illustrates that a more accurate ρR value can be ablator, generating 4.44 MeV gamma-rays equation 3.17 assigned to the dsr value, if the effect of the primary- that are emitted quasi-isotropically. Along the same neutron-source and high-density profile, both 3D in discussion as for down-scattered neutrons and up-scattered nature, are known (see discussion in section 4.8 and ions, the yield ratio between 4.44 MeV gamma-rays and ρ [50]). In this case, the fuel density is not integrated primary neutrons carries information about RCH as radially, but rather over the density-weighted primary- described by 〈 〉 neutron path-length distribution L through the fuel. ()b + 12 mp Yg This applies to all yield-ratio methods. In addition, a rRCH = .4.15() s 12 Y more complete picture of the assembled fuel can be nC* n obtained with dsr values assigned to different portions of the implosion. Here, snC12 * is the inelastic cross section for generating 2. Yield ratio between up-scattered ions and primary neutrons 4.44 MeV gamma-rays and Yγ is the yield of 4.44 MeV As illustrated by the scattering processes (3.13)– gamma-rays from the first, excited state of 12C. In the (3.15), up-scattered ions (deuterons, tritons or protons) are case of an HDC ablator, β=0. Given that the gamma- rays are emitted quasi-isotropically, this expression 8 The down-scattered-neutron spectrum in the 10–12 MeV range is dictated represents a spatially-averaged ablator ρR. by the elastic-scattering process. 4. Relative yield of un-scattered primary neutrons emitted 9 The coefficient in front of the second term is incorrect in [50]. 10 For an equimolar DT implosion, with no ablator remaining, the single- in different directions down-scatter-neutron yield Yn′ scales as (σnd/5mp) ρRYn, while the double- As the yield of down-scattered neutrons depends ∼ ( 2 / ) 2 down-scatter-neutron yield Yn″ at 11 MeV scales as sdt 20mp rRdt Yn. on ρR, the yield of un-scattered primary neutrons ∼ ∼ This means that the double-down-scatter process removes 10% and 20% (Y ), above certain energy, also carries information of the single-down-scattered neutrons in the energy range of 10–12 MeV for unsc −2 about the ρR of the region traversed by the primary a ρRdt of 1 and 2 g cm . Here, we have neglected that the source dt distributions of n and n′ are different. neutrons (the effect of the ρRCH is small and can be

9 Plasma Phys. Control. Fusion 62 (2020) 023001 Topical Review neglected in this discussion). This can be shown by first extensively in the literature [69–73], the challenge is to expressing Yunsc in terms of the yield difference determine the functional form of both G(vi/vth) and between produced primary neutrons and down-scattered lnΛe, which depend on the ion velocity and plasma neutrons conditions. ⎛ ⎞ 6. Yield ratio between secondary dt neutrons (or d3He asnd+ s nt ) YYYunsc=- n n¢ =⎜1 - rRYdt⎟ n,4.16() protons and primary dd neutrons ⎝ ()23a + mp ⎠ Given that the velocities of the 1.01 MeV tritons and 0.82 MeV 3He ions (produced by the dd reactions shown by σ σ where nd and nt are the effective nd and nt elastic- equations (3.4)–(3.5)) are comparable to vth in a typical ICF scattering cross sections, respectively, for generating implosion, the energy loss depends strongly on ρRd, Te and down-scattered neutrons below a certain energy. For an plasma composition in an ICF implosion. This means that the α= / = ( ) equimolar dt fuel, nd nt 1, and equation 4.16 yield ratios between secondary dt neutrons and dd neutrons / 3 can be reformulated in terms of Yunsc Yn (Y2n/Yn) and secondary d He protons and primary dd neu- ⎛ ⎞ trons (Y2p/Yn) provide information about these parameters, 5mp Yunsc rRdt = ⎜1- ⎟ .() 4.17 which has been discussed extensively in the literature by ss+ ⎝ Y ⎠ nd nt n Azechi et al [74–76], Cable et al [77], Séguin et al [78], and Kurebayashi et al [79]. Recently, Rinderknecht et al extended Looking at this quantity in several emission directions this work by demonstrating that the combined information allows for the generation of an un-scattered neutron- from Y2n/Yn and Y2p/Yn provides information of at least two yield map or a ρRdt map. As it is challenging to directly of above-mentioned plasma parameters [80]. The reason for ρ quantify Yn in high- R implosions, a spatial average of this is that 〈σvrel〉dt increases and 〈σvrel〉d3He decreases as the 3 the un-scattered yield 〈Yunsc〉 is more practical to use in tritons and He ions lose energy in the plasma, respectively. equation (4.17). Replacing Yn with 〈Yunsc〉 makes the Given that the range of the tritons is also ∼10× longer than 3 diagnosed ρRdt a relative quantity. that of He ions, the combined information provided by the 5. Energy loss of charged fusion products two yield ratios elucidates unique information about the hot −2 While yields of down-scattered neutrons or up- spot for ρRd values up to ∼100 mg cm . In addition, as the scattered ions, relative to the primary yield, carry range of the 1.01 MeV tritons and 0.82 MeV 3He ions information about ρR, energy loss of primary and depends strongly on the magnitude of the plasma stopping secondary charged fusion products carry information power, which approximately scales as 1/Te, an increased Te about ρR as described by reduces the stopping power, leading to increased ion ranges dR()r 1 and increased probabilities for the secondary fusion reactions rR =-dE =- dE,4.18() / / [ – ] òòdE 1 dE and higher Y2n Yn and Y2p Yn 74 80 . r dR 4.7. Electron temperature where dE/dR (or dE/dX in planar geometry) represents − For fuel ρR values above 100 mg cm 2, both Y /Y and the rate of energy loss per unit distance traversed by the d 2n n Y /Y generally saturate. In this regime, the yield ratio pro- charged fusion product (called plasma stopping power). 2p n vides information about T of the region traversed by the In general, the plasma stopping power is expressed in its e tritons and 3He ions assuming mix of high-Z material into the most basic form as [68] fuel is insignificant. dE ⎛ Ze⎞2 ⎛ v ⎞ =-⎜ i ⎟ w2 G⎜ i ⎟ ln L ,() 4.19 ⎝ ⎠ p ⎝ ⎠ e dX vi vth 4.8. 3D asymmetries The discussion in section 4 has so far been limited to 1D 2 1/2 where ωp=(4πnee /me) is the electron plasma observables. Understanding the impact of 3D asymmetries on frequency, ne is the electron-number density, Zi and vi the implosion performance is also an essential prerequisite for are the charge and velocity of the charged fusion obtaining a complete picture of the fuel assembly and hot- product, respectively, vth is the average velocity of the sport formation, because a non-spherical assembly of the thermal electrons, lnΛe is the Coulomb logarithm that is main fuel reduces the efficiency of converting shell kinetic a central parameter determining the interplay between energy to hot-spot thermal energy at stagnation, leading to long and short distance Coulomb interactions and thus lower hot-spot pressure P and reduced confinement τ [81]. the characteristics of the plasma stopping power. A few The 3D information can be accessed by diagnosing the qualitative statements can be made. First, dE/dX is implosion along different LOS. An example of 3D data is the practically independent of Te for high-energy primary or spatial variation in the first and second moments in the pri- secondary fusion products with velocities vi much mary fusion-product spectra. Large variations in these para- higher than vth, enabling ρR to be uniquely determined meters are indicators of turbulent macroscopic flows in the from the energy loss. Secondly, dE/dX depends hot spot, significantly affecting VCM in the laboratory system. strongly on both ne and Te when vi∼vth. As described Other examples of 3D asymmetries are spatial variations in

10 Plasma Phys. Control. Fusion 62 (2020) 023001 Topical Review the nuclear burn profile, dsr, and Yunsc, which indicate a non- Given that a particular diagnostic has its own set of spherical hot spot and spatial ρR variations in the high-density limitations, different types of diagnostics are commonly used fuel shell surrounding the hot spot. for measurements of a certain implosion parameter. In this section, we present the neutron diagnostics that are, or have been, routinely used to measure the parameters relevant to the 4.9. Kinetic and multi-ion effects nuclear phase of an ICF implosion. When a converging shock rebounds at the center of an ICF implosion, it significantly raises Ti and ni and initiates the 5.1. Neutron activation diagnostics nuclear burn. At this time, the mean-free path for ion-ion The main objective with the Neutron Activation Diagnostic collisions is sufficiently long, relative to the plasma-scale (NAD) is to measure the absolute yield of neutrons emitted in length, that both the shock front itself and the resulting hot a certain direction from an ICF implosion11. Generally, a spot are governed by kinetic and multi-ion physics. Under NAD, made of a suitable element, is positioned at a known these conditions, the combined information of absolute pri- distance from the implosion and is irradiated by the emitted mary yields, primary-yield ratios, T and burn profiles carry i neutrons. When neutrons, above a certain reaction threshold, information about kinetic and multi-ion effects in an ICF interact with the chosen diagnostic material element, nuclear implosion. These phenomena can be explored by comparing reactions occur that create a radioactive isotope of the ele- average-ion hydrodynamic simulations to experiments ment. After the neutron exposure, the NAD is removed and [82–86]. In addition, nuclear burn profiles and burn histories brought, either manually or automatically, to a counting sta- carry information that goes beyond zero-dimensional, burn- tion where the subsequent decay of the radioactive isotope is integrated quantities, providing spatially and temporally measured, from which the number of neutrons passing resolved information about kinetic and multi-ion effects [87]. through the NAD can be quantified. The absolute yield of emitted neutrons, above a certain reaction threshold, is 4.10. Magnetic and electric fields determined by 2 fi 4pdANg Strong electric and magnetic elds often persist in plasmas Yunsc = .5.1() -Dlltt12 -D relevant to ICF [88–92], and information about these fields is mffNeNAD BR a Adeffes ()1 - e carried by the primary charged fusion products, generated in Here, d is the detector distance to the implosion, A is the mass reactions (3.1)–(3.12). As discussed in some detail number of the isotope used for the detection, Nγ is the number section 7.7, the direction of the incoming and outgoing quasi- of measured gamma-rays emitted by the decay, mNAD is the mono-energetic charged fusion products, when produced by mass of the NAD, fBR is the branching ratio producing the an external point source, carry information about the electric gamma-rays of interest, fa is the abundance of the isotope in and magnetic fields in direct-drive, cone-in shell, and indirect- the NAD, NA is Avogadro’s number, εd is the detection drive implosions. efficiency, σeff is the neutron-spectrum-weighted effective cross section, λ is the decay time constant of the radioactive isotope, Δt1 is the time between the implosion time and start Δ 5. Neutron diagnostics of the radioactive-decay measurement, and t2 is the time duration for the radioactive-decay measurement. It should be To probe the wealth of information carried by neutrons, a noted that the effects of neutron scattering and absorption by wide range of neutron diagnostics have been implemented nearby structures and the NAD itself are not considered by and extensively used at the different ICF facilities worldwide. equation (5.1) [93]. In addition, for an accurate assessment of σ Given that fusion yields have increased several orders of eff and yield of the emitted neutrons, the data must be ana- magnitude since the initiation of the experimental ICF pro- lyzed using the actual neutron spectrum from the implosion. gram in the early 70s, the quality of the neutron measure- Historically, the analysis has used a Gaussian primary-neu- ments has improved significantly, facilitating an increased tron spectrum and thus not considered the down-scattered- level of understanding about the physics governing the neutron component above the reaction threshold, which can nuclear phase of an ICF implosion. At the biggest facilities, have an impact on the analysis. such as the NIF, fusion yields are now high enough for In the case of low neutron yields, a high-efficiency sys- neutron measurements to provide spatial, temporal and tem is required. This means that a NAD with a large mass spectral information, which have proven indispensable to (tens of grams) must be used, which complicates the absolute understanding the performance of an ICF implosion. At the calibration of the detector, as the fluence of the neutrons same time, the requirements on these measurements have also through the NAD changes and self-absorption of the gamma- become more stringent. Fusion yields need to be measured rays emitted by the system may be significant. These issues over several orders of magnitude, up to ∼2×1016 or ∼50 kJ are typically addressed by exposing the NAD to a known of fusion energy; ion temperatures need to be measured with high accuracy up to ∼10 keV; nuclear burn profiles and 11 This should be distinguished from the absolute neutron yield Y produced temporal histories need to be measured with spatial and n in a high-ρR implosion, as the unscattered primary-neutron yield 〈Yunsc〉 is temporal resolution of order 10 μm and 10 ps, respectively. significantly less than Yn.

11 Plasma Phys. Control. Fusion 62 (2020) 023001 Topical Review

11 Figure 5. (a) The copper-activation system on OMEGA-60. (b) Thick Cu activation, which is used for low-yield implosions (Yn<10 ), and 11 thin Cu activation, which is used for high-yield implosions (Yn>10 ). neutron-fluence spectrum generated by a neutron generator in uncertainty in the measured un-scattered yield has been which the neutron production is monitored with the asso- estimated to be ∼7%, dominated by uncertainties in the ciated-particle technique [94]. calibration and counting-geometry corrections (estimated to In the following sections, the different types of NADs be ∼5%). Uncertainties associated with absorption and routinely used in the field of ICF are reviewed. For more scattering in nearby structures play a role. On the other detailed discussions about the NAD diagnostics, the reader is hand, the 90Zr(n,2n) cross section is known to a 1% referred to the papers by Barnes et al [95], Bleuel et al [96], uncertainty and does not significantly impact the total and Yeamans et al [97]. uncertainty. It has also been demonstrated that a mono- energetic dt-neutron spectrum and a Doppler-broadened dt- Δ = 5.1.1. Copper. Copper has been used as NAD material since neutron spectrum with Ed 350 keV generate spectrum- the beginning of the experimental ICF program for weighted cross sections of 596 mb and 595 mb, respectively, Δ [ ] measurements of the emitted DT neutron yield [98, 99]. demonstrating an insensitivity to changes in Ed 96 . The technique is based on the 63Cu(n,2n)62Cu(β+) reaction The advantage of using zirconium as opposed to copper that has a threshold of 10.9 MeV. As 62Cu is radioactively is that the reaction threshold is higher, which makes the unstable, with a half-life of 9.8 min, it decays into 62Ni system less sensitive to down-scattered neutrons and thus through positron emission. The emitted positron annihilates more ideal to probe un-scattered neutrons emitted in various with an electron and produces two 0.511 MeV gamma-rays directions. As the yield of un-scattered primary neutrons carry emitted in opposite directions. The Cu-activation system on information about the ρRdt of the region traversed by the OMEGA-60 is schematically shown in figure 5(a), where two primary neutrons (equation (4.17)), a relative ρRdt map can be NaI inorganic scintillators are used to detect the emitted generated from data obtained with several NADs located gamma-rays. Similar Cu-activation systems exist at the Z- around the implosion. These maps often show angular facility [100], the NIF [101] and LMJ [102, 103]. variations consistent with low-mode ρR asymmetries. On With a threshold of 10.9 MeV, the Cu-activation about half of the high-foot implosions [105] at the NIF, the measurement is sensitive to both down-scattered and primary FNADs data are fitted with spherical harmonic distributions dt neutrons. In the case of high-ρR implosions at the NIF, with modal numbers up to L=2, which show regions of high down-scattered neutrons with energies above the reaction ρR at the poles (see example in figure 6(a)). The other 50% of threshold will affect the yield measurement, and according to the data show residual RMS deviation from the L=2 fit, calculations [99], typical neutron spectra at the NIF, with a implying the existence of L-mode numbers >2. In implosions significant down-scattered-neutron component, have been of capsules not held with a thin membrane inside the predicted to generate a ∼10% lower induced activity than a hohlraum, a high ρR feature are often observed close to the mono-energetic dt-neutron source. fill-tube axis suggesting that at least in the case of a 30 μm diameter fill tube, the impact on the shell symmetry is fi ( ρ 5.1.2. Zirconium. Zirconium is used as activation element in signi cant an example of a R perturbation possibly due to the most NAD systems on the NIF for measurements of the fill tube is shown in figure 6(b)). mainly un-scattered primary dt neutrons [96, 97, 104]. For the The disadvantage of using Zirconium instead of Copper detection, the 90Zr(n,2n)89Zr reaction is utilized, and this is that the half-life is much longer (3.27 d versus 9.8 min), reaction has a reaction threshold of 12.1 MeV and cross which means that it takes a longer time to collect data with section that gradually increases up to ∼0.8 b at 15 MeV. The good statistics. This is not adequate at OMEGA-60 and other 89Zr has a 3.27 day half-life and β+ decays to 89mY, which smaller ICF facilities where the shot cycle is about an hour, subsequently emits a 909 keV gamma-ray. The gamma-rays while it is acceptable at the NIF operating at a lower shot rate. are measured with lead-shielded high-purity germanium In addition, relatively high ρRdt and ρRdt asymmetries are detectors in a low-background counting facility. The required for this method to be effective.

12 Plasma Phys. Control. Fusion 62 (2020) 023001 Topical Review

Figure 6. Yunsc/〈Yunsc〉 ratio measured with several zirconium NADs (positions indicated by the open circles) for NIF implosions (a) N150422 and (b) N160628. Smaller black data points indicate the positions of the nTOF and the MRS neutron spectrometers used for the dsr measurements (discussed in sections 5.2 and 5.6), from which directional ρR values are determined. The color map was generated by fitting a low-mode spherical harmonics to the data. The data shown in (a) illustrate high ρR values at the poles, while the data in (b) show a high ρR feature at the location of the fill tube. This mass perturbation and associated radiation shadowing may be seeding a large defect that could perturb the stagnated high-density shell. A Yunsc/〈Yunsc〉 variation of about ±10% roughly corresponds to a ρR asymmetry of about ±500 mg cm−2.

5.1.3. Indium. For detection of dd neutrons (equation (3.4)), spectrum [115, 116]. For substantial coverage of the implo- indium is used as activation element in which the 115In(n, sion, a suite of nTOF spectrometers with different LOS and n′)115mIn reaction is utilized [101]. As the 115In(n, n′)115mIn distances to the implosion are typically used to probe different cross section increases rapidly between 1 and 2 MeV and aspects of the ICF neutron spectrum. remains relatively constant around 2.45 MeV, it is quite The nTOF spectrometers are generally based on a cur- insensitive to Ti variations, or rather ΔEd variations. On the rent-mode system consisting of a detection medium, either other hand, due to low dd-neutron yields, a relatively low made of a scintillator or chemical vapor deposition (CVD) reaction threshold and a higher sensitivity to scattered diamond (new materials, such as quartz crystal is also being neutrons, the indium sample must be positioned close to the implemented as discussed in section 9.1). In the case of a implosion, where the mass of nearby structures is minimal. scintillator, it is optically coupled to a photomultiplier tube The 115mIn isomer emits 336.2 keV gamma-rays with a 4.5 h (PMT) or a vacuum photodiode (PD). High-bandwidth digital half-life, and detection of these gamma-rays typically results oscilloscopes are used to record the nTOF signal trace s(t), in a yield uncertainty of ∼10%. Indium has been used which can be described by routinely in the past at both OMEGA-60 and the NIF. Today dE it is used for dd-neutron yield measurements at the Z-facility st()= f [ Et ()]·e [ Et ()]· aEt [ ()]IRF () t ,5.2 ( ) [100] and Shenguang-II facility [106, 107]. dt where f [E(t)] is the neutron spectrum, ε[E(t)] is the energy- 5.1.4. Other material. In addition to the standard activation dependent efficiency of the nTOF system, a[E(t)] is the beam- materials, carbon has been explored as an activation element line transmission factor, E(t) is the neutron energy versus for tertiary neutrons above 22 MeV [108]. As briefly touched 2 time-of-flight and expressed as E(t)=mnc (γ−1), where mn upon in section 3.3, the motivation for this measurement was is the neutron mass, γ=(1−β2)−1/2, and β=tγ/t with tγ ρ to determine the total R of an implosion using the yield ratio being the time-of-flight of a photon. dE/dt converts the between tertiary and primary neutrons. To accurately measure neutron spectrum from energy to time-of-flight and is the yield of tertiary neutrons, it was concluded that the carbon 2 3 3 expressed as mnc γ β /tγ, and IRF(t) is the instrument sample cannot contain contaminants more than one part per response function. The nTOF signal trace is typically ana- million for this technique to work. In particular, the air with lyzed by using a forward fit of the data (deconvolution is 14 N must be removed from the carbon material. sometimes used), which involves an analytical description of the neutron spectrum and the different factors in 5.2. nTOF spectrometers equation (5.2). Examples of nTOF signal traces that have been analyzed using the forward-fit technique are shown in For decades, nTOF spectrometers have been used extensively 12 figure 7. From the measured dt signal trace, Y can be dd dt unsc for measurements of the primary or neutron spectrum at determined by integrating the signal over the time window most large-scaled ICF facilities around the world [109–113]. corresponding to a neutron-energy range of 13–15 MeV. As In addition, nTOFs are now being used for measurements of [ ] 12 the down-scattered neutron spectrum 114 , primary neutron Given that the primary-neutron signal is integrated from 13 to 15 MeV, spectrum (the first three moments) [59, 114], and tt neutron Yunsc discussed here is slightly different from the zirconium-measured Yunsc.

13 Plasma Phys. Control. Fusion 62 (2020) 023001 Topical Review

Figure 7. Example of nTOF-signal traces for (a) dt-primary and down-scattered neutrons, and (b) dd-primary neutrons that is recorded on top of the down-scattered dt neutron signal. These signal-traces were obtained with one of the nTOF spectrometers positioned ∼20 m away from the implosion at the NIF. Reproduced from [118]. © IOP Publishing Ltd. All rights reserved. this is a relative measurement, an absolute yield calibration for an nTOF spectrometer. To convert this x-ray-IRF to a must be obtained by using other absolute Yunsc measurements. neutron-IRF, the neutron transit time through detecting When calibrated, the primary-yield accuracy is typically medium must be taken into account in the analysis. This is ∼7%, while the dsr accuracy is ∼10%. The modeling of done by using neutron-transport simulations, in which the nTOF data is described in detail in [117]. time duration of the neutron-induced charged-particle energy Positioning an nTOF spectrometer at a large distance deposition is determined and convolved with the x-ray IRF. from an implosion, while at the same ensuring the solid angle and efficiency are adequate, Ti (the second moment) can be determined from the spread of the neutron arrival times at the 5.2.1. Scintillator-based systems. When neutrons interact detector, which is dictated by the neutron velocity distribu- with a scintillator they generate charged particles through tion. This spread is caused by Doppler broadening and elastic-scattering processes or nuclear reactions, and these macroscopic flows in the plasma, as discussed in sections 4.2 charged particles deposit their energy in the scintillator and produce light mainly in the visible range. The light is and 4.3.Ifflows are insignificant, Ti can be expressed as subsequently transported to a PMT where it is converted to an 2 2 DtD electrical signal that is amplified. At high neutron yields, the Tci = 1 ,keV() () 5.3 d2 PMT is often replaced with a lower-gain PD. The first nTOF spectrometers implemented on ICF facilities were generally where ΔtD is the Doppler spread (FWHM) of the neutron- arrival-time distribution in nanoseconds at the detector, d is on-axis systems where the PMT was directly exposed to the incoming neutrons. This was not optimal from a signal-to- the neutron flight path in meters, and c1=1.3 for dd neutrons background (S/B) point of view, as unwanted background and c1=7.8 for dt neutrons. For a given neutron-arrival-time was generated in the PMT. More recent nTOFs are off-axis spread and nTOF distance, Ti is 36 times larger for dt neu- trons than for dd neutrons. The accuracy in the Ti measure- systems, where the PMT is positioned behind a shielding and ment is generally in the range of 100–300 eV. outside the collimated beam of incoming neutrons. To obtain To determine the energy-peak shift (first moment), the highest possible signal-to-noise (S/N) ratio, a scintillator is time offset of the measured signal must be known to very high often viewed by up to four off-axis PMTs/PDs. The signals accuracy. As discussed in [118], the absolute timing of the from these are recorded on several oscilloscope channels with detected signal is affected by several terms as described by different sensitivity settings, and the data obtained are often stitched together to generate a complete signal time trace that tt=+ t + t + tEt() + ,5.4 ( ) 0 offset BT E fidu includes both the primary and down-scattered-neutron where t0 is the time between the rise of the drive and actual signals. Examples of off-axis nTOF spectrometers on time when the drive (laser or magnetic drive) hits the target, OMEGA, the NIF, and the Z-facility are shown in figure 8. toffset is the time offset that has to be determined from an x-ray Fast plastic scintillators were initially used for the nTOF timing shot, tBT is the bang time (peak nuclear burn), tE(E) is spectrometers on most ICF facilities, but at OMEGA and the the energy-dependent neutron-time-of-flight time from the NIF these plastic scintillators were replaced by liquid Xylene implosion to the nTOF spectrometer, and tfidu is the position of scintillator with a faster decay time [110]. At the NIF, the the timing fiducial on the oscilloscope trace. At the NIF, t0 has Xylene detectors were in turn recently replaced by Bibenzyl been established to be ∼30 ps. crystals [119, 120], which are even faster than the liquid X-ray timing shots, which produce x-rays over a short Xylene. This feature made it possible to reduce the mass in period of time, are routinely used to establish the x-ray-IRF the nTOF spectrometers, as illustrated in figure 8(b), which

14 Plasma Phys. Control. Fusion 62 (2020) 023001 Topical Review

Figure 8. Off-axis nTOF spectrometers on (a) the NIF, (b) OMEGA-60 and (c) the Z-facility. A collimated beam of neutrons hits the scintillator (marked yellow in (a) and (b)), which is observed by a set of off-axis PMTs/PDs positioned around the scintillator. In the case of OMEGA-60, a smaller off-axis system is also positioned in front of the bigger nTOF spectrometer.

Figure 9. The nTOF spectrometers on the NIF for measurements of primary and down-scattered-neutrons. These spectrometers are positioned ∼20 m from the implosion, and each system consists of collimators, a scintillator, and off-axis PMTs/PDs that are shielded. Each LOS is specified in the figure by the polar-azimuthal angles. Additional nTOFs for primary and down-scattered neutron measurements have been fielded at 90°–315° and close to the North Pole. The magnetic recoil spectrometer (MRS), positioned on the 10 m diameter chamber, is also shown. The MRS is discussed in detail in section 5.6. resulted in a lower background of scattered neutrons. The interact with the material they generate electron–hole pairs locations of the ∼20 m nTOF spectrometers on the NIF for that are drawn out by a bias field, which produces a time- both primary and down-scattered neutron measurements are dependent voltage that is recorded on an oscilloscope. shown in figure 9. More detailed discussions about nTOF As the CVD diamond detector is compact and vacuum- systems on OMEGA-60, the NIF, the Z-facility, LMJ, Gekko- compatible, it can be fielded close to an implosion to XII and Shenguang III can be found in [109–113]. maximize signal statistics and minimize particle time-of-flight spread, making it ideal for yield and bang-time measure- ments. Given that the CVD sensitivity to protons is about 5.2.2. Chemically-vapored-deposition (CVD) diamond three orders magnitude higher than for neutrons, when detector. A CVD diamond detector is ideal for measurements of neutrons, gamma-rays and charged considering interaction probabilities, it is used for simulta- particles. Given its fast time response (of ∼200 ps) and neous measurements of shock- and compression-bang times 3 (∼ 8) (∼ 11) relatively low efficiency, it has been used routinely for using d He protons 10 and dd neutrons 10 , 3 measurements of primary neutrons in high-yield applications respectively, in D He gas-filled surrogate experiments at the at OMEGA-60 [121] and for measurements of bang time in NIF. This diagnostic is called particle time-of-flight (pTOF) implosions at the NIF [122]. The CVD is made of a wafer that diagnostic [122], and is illustrated in figure 10. An example of is biased along its axis, and when high-energy particles pTOF data is shown in figure 11. With a fast rise time, the

15 Plasma Phys. Control. Fusion 62 (2020) 023001 Topical Review

Figure 10. (a) Components of the CVD diamond detector used in the pTOF diagnostic on the NIF. The insert illustrates the assembled system. (b) A CAD model of the pTOF diagnostic mounted on an external bracket attached to a diagnostic insertion manipulator (DIM). Three wedge range filter (WRF) spectrometers, discussed in detail in section 7.2, are also mounted externally on the 90°–78° DIM.

energy tail of the primary dd-neutron spectrum [129]. Another single-hit spectrometer was the Tion diagnostic that was implemented by Los Alamos National Laboratory (LANL) on NOVA. This system consisted of 1020 detectors each made up of a scintillator and photomultiplier tube. The Tion array was ∼100× less sensitive than LaNSA and thus could operate at higher yields. In particular, the capability to measure the leading edge of the neutron-arrival-time distribution (before it saturated by the bulk of the primary neutrons). It was also used to characterize mix in ICF implosions either by measuring the yield from deuterated plastic capsule Figure 11. Example of a pTOF signal trace from NIF shot N150126. implosions, or energy spectra of secondary neutrons This trace shows the hohlraum x-rays, d3He protons originating from produced in medium-converging implosions [130]. the shock-burn phase, and dd neutrons from the subsequent compression phase. During the compression phase, the ρR is often high enough to range out the d3He protons. At ~46 ns, the negative timing fiducial signal is observed. 5.3. Nuclear burn-history detectors The nuclear burn history, or the fusion reaction rate, was first signal is measured with a timing accuracy that is better measured in the 90s with a Neutron Temporal Diagnostic ± than 50 ps. (NTD) on NOVA [131]. Using the experience gained with this diagnostic, another NTD was built and optimized for 5.2.3. Single-hit spectrometers. Single-hit spectrometers OMEGA-60 about a decade later [132]. Building upon the have been used extensively for low-yield applications at NTD technique, the Particle Temporal Diagnostic (PTD) was NOVA [123, 124], OMEGA-60 [125] and Gekko XII [126]. subsequently implemented on OMEGA-60 for measurements These systems, which are high-efficiency spectrometers, of primary dd and d3He burn histories as well as the sec- consisted of an array of scintillators, each coupled to a ondary d3He burn history [133], followed by the imple- PMT, followed by adiscriminator, Time-to-Digital Converter mentation of the Particle x-ray Temporal Diagnostic (PXTD) and Analog-to-Digital Converter that were used to record for simultaneous measurements of multiple burn histories and theneutronarrival time and pulse height. From the x-ray emission history (in different energy bands) [134]. distribution of the neutron arrival times, a neutron spectrum These systems are all based on a fast organic scintillator could be constructed. At NOVA, the large area neutron (BC-422) that is positioned close to the implosion for mini- scintillator array (LaNSA) comprised of 960 scintillators that mization of the particle time-of-flight broadening (see were positioned 20 m away from the implosion [123]. This equation (5.3)). Thin scintillators are also used to minimize system was capable of measuring secondary dt-neutron yields the transit time of the particles across the thickness of the as low as ∼2×105 (100 detected hits) with a resolution of scintillator for optimal time resolution. When neutrons, 2.3 ns (or 170 keV for 14 MeV neutrons with a flight path of charged particles and x-rays interact with the scintillator, light 20 m), from which a fuel ρR in D2 gas-filled, low-CR is generated that is collected by a collection lens. This light is implosions was determined [127, 128]. As the system had subsequently transported through an optical-relay system to very high efficiency, the primary dd-neutron spectrum could an optical streak camera, positioned either outside the target not be measured because of too many channels were chamber or behind the target-bay wall. A schematic of the triggered. However, LaNSA was used to diagnose the high- PTD is illustrated in figure 12.

16 Plasma Phys. Control. Fusion 62 (2020) 023001 Topical Review

Figure 12. Schematic of the PTD on OMEGA-60. This system uses a 3.5 m optical relay to transport light from the scintillator positioned at 9 cm from the implosion to an optical streak camera outside the target chamber. The timing-fiducial system provides cross-timing between the signals and incident laser pulse, which are recorded on the P510 UV streak camera. A zoomed-in schematic view of the PTD front end is also shown, which consists of a 1 mm thick scintillator with 100 μm tantalum filter in front. Part of this figure is reprinted from [132], with the permission of AIP Publishing.

As the BC-422 scintillator has a fast rise time (<20 ps) levels well within the dynamic range of the optical streak and long decay time (∼1.4 ns), the information of the nuclear camera. The reason for this is that dd neutrons produce burn history is encoded in the leading edge of the streaked ∼600× less photons than d3He protons. This is due to the fact signal image. To obtain this information, the effect of the that a small fraction of neutrons interacts and deposits energy scintillator decay is deconvolved from the recorded signal, in the scintillator, whereas every d3He proton deposits energy and the model used for this is given by in the scintillator. In the case of Bremsstrahlung x-rays, the fi i-1 signal levels vary signi cantly depending on the energy band. ()ij-D tp STii=-å Sej t .5.5()For a robust simultaneous measurement of these emission j=0 histories, the signal levels must be within a factor of five, which is accomplished by adjusting the light-attenuation fil- Here, S is the measured signal at the pixel location i, which is i ters behind each scintillator. Figure 14 illustrates examples of the difference between the total signal T and the sum of i streak images from the 2-channel or 3-channel PXTD. previous signals that decay exponentially. In addition, Δtp is the time separation of two pixels. As discussed in [132], this method is fast, reliable and insensitive to noise in the streak 5.4. Imaging systems camera signal. Although the scintillator is positioned close to the implosion, the temporal broadening of the signal due to A neutron-imaging system typically consists of a pinhole spectral broadening must also be considered in the decon- (and/or penumbral aperture), a mechanical alignment system volution process. for the pinhole/aperture, and an imaging detector. Due to the An example of a raw PTD streaked image and projected penetrating nature of the neutrons, the pinhole/aperture must primary-dd and secondary-d3He signals are shown in be very thick, which results in a narrow field of view and very figure 13. For an absolute timing reference of the streaked tight alignment tolerances. The detector typically consists of image, timing fiducials are used to generate a series of light either a bundle of scintillation fibers or capillary tubes filled pulses spaced 548 ps apart (see top-part of the image), which with a liquid scintillator. Either penumbral or pinhole imaging is delivered via an optical fiber. The fiducial pulses are also (or in some cases both techniques) are used to image an ICF used to correct for any sweep nonlinearities. implosion [135]. Penumbral imaging involves a coded aper- To enable measurements of multiple nuclear-reaction and ture and conceptually it is similar to pinhole imaging, with the x-ray-emission histories (in different energy bands), the essential difference that the aperture is larger than the source. PXTD was recently implemented. This system uses either two The detected image consists of an umbra and a penumbra. or three scintillators combined with more complex filtering. The former region is exposed to the entire source through the The main challenge with this diagnostic is to keep the signals aperture, while the latter is progressively obscured from the

17 Plasma Phys. Control. Fusion 62 (2020) 023001 Topical Review At OMEGA-60, the Commissariat à l’Énergie Atomique (CEA) fielded and successfully used a penumbral imaging system combined with a detector based on 85 μmdiameter capillary tubes filled with a liquid scintillator [137]. This was done in preparation for the implementation of a system on the LMJ laser. Figure 15 shows a schematic of the imaging system on OMEGA-60. The first reincarnation of the detector had a resolution of ∼650 μm (FWHM) at 14 MeV, which was dic- tated by the track length of elastically-scattered recoil protons in the scintillator material. With this system, images were recorded with a resolution of ∼20 μm and signal-to-noise (S/ N) ratio of ∼40 [138].Imagesofdd neutrons from D2 gas- filled capsule implosions were also recorded with this system. A one-dimensional neutron imager, called ODIN, has been implemented on the Z-facility for imaging dd-neutrons [139]. This system is primarily intended for imaging MagLIF cylindrical implosions that are about 10 mm long and 100 μmin diameter. As discussed in the paper by Ampleford et al [139], ODIN system uses an extended, rolled-edge slit that is centered 22 cm from the implosion. CR-39 is used as the principle detector that is positioned 79.5 cm from the slit, providing a modest magnification of 3.6. This system is designed to provide a500μm axial resolution over the entire implosion length with good signal-to-noise ratio for dd-neutrons yields above 1012. The key components of ODIN is shown in figure 16. At the NIF, the first neutron-imaging system has been Figure 13. (a) PTD streak image for a D2 gas-filled capsule implosion (shot 38309). This image illustrates secondary d3He- implemented and extensively used to diagnose the size and proton and primary dd-neutron signals. Due to a higher velocity, the shape (in particular low-mode asymmetries) of the hot-spot d3He protons arrive at the scintillator before the dd neutrons. The and surrounding the high-density shell [140]. This is done by fi ducials appear at the top of the streak image and are used for imaging primary dt-neutrons and down-scattered neutrons absolute timing relative to the laser pulse, and for possible ( ( ) that mainly scatter within the surrounding high-density fuel adjustment of any sweep non-linerarities. b Projected signal on the ) time axis. Both signals are sitting on top a decaying x-ray shell . To separate these neutrons, the detector system is gated background. to record primary neutrons with energies between 13 and 17 MeV and down-scattered neutrons from 6 to 12 MeV. For a high-quality image of the down-scattered neutrons, which source by the aperture edges. The information about the are recorded after the primary neutrons, the scintillator output source is therefore encoded in the penumbra. from the primary signal must decay at least three orders of In the case of a penumbral aperture, the neutron transmis- magnitude before the lower-energy neutrons can be recorded. sion through the material in the aperture determines the spatial The imaging system on the NIF, shown in figure 17, resolution. In the small-angle approximation, the spatial blurring consists of an array of pinholes and penumbral apertures (see of the point spread function is dictated by the field-of-view insert in figure 17) positioned 26.5 cm from the implosion, and and aperture-to-implosion distance (L ). When considering the 0 ascintillatingfiber array positioned 28 m from the implosion, detector resolution (ΔS ), the total resolution (ΔS ),repre- det tot which provides a magnification of ∼85 [141, 142].Thissystem sented by the FWHM, is given by was designed to provide a spatial resolution in the vicinity of 2 2 μ 15 ⎛ln 2 · FOV ⎞ ⎛ L ⎞ 10 m for yields above 10 . Examples of primary and down- D=S ⎜ ⎟ +DS2 ⎜ 0 ⎟ ,5.6() tot det ⎝ ⎠ scattered neutron images are shown in figure 18. ⎝ 2mL0 ⎠ Li where μ is the attenuation of the neutrons through the aperture 5.5. CR-39 detectors material and Li is the detector-to-implosion distance. The first neutron-imaging system was implemented on CR-39 is a transparent plastic with a chemical composition of NOVA in the 80s [136]. This system was based on a C12H18O7, and this material has been used to detect neutrons, penumbral coded aperture, and provided the first neutron from which an absolute yield of primary neutrons emitted from images of an ICF implosion. In these measurements, shock- an ICF implosion was determined [143]. The CR-39 detection driven implosions were imaged with a limited spatial reso- of neutrons is a two-step process. The first step is the generation lution (∼60 μm), dictated by the relatively low magnification of an energetic charged particle in the CR-39, either through and long range of the elastically-scattered recoil protons in the elastic scattering or nuclear reaction. This charged particle scintillator. leaves a trail of damage along its path in the CR-39 in the form

18 Plasma Phys. Control. Fusion 62 (2020) 023001 Topical Review

Figure 14. (a) 2-channel PXTD streak image of the primary dd-neutron and d3He-proton signals. This data was generated by a D3He gas- filled (with trace argon) capsule implosion (shot 77119). Due to a higher velocity, the d3He protons arrive at the scintillator ∼2 ns before the dd neutrons. (b) Projected d3He-proton and dd-neutron signals on the time axis. (c) 3-channel PXTD streak image of the x-ray signal in three energy bands. This data was obtained from a T2 gas-filled (capsule implosion (shot 80705)). (d) Projected x-ray signals on the time axis. Reprinted from [134], with the permission of AIP Publishing.

of tracks in the CR-39, and thus the CR-39 neutron detection efficiency, must be understood. Both dd and dt neutrons can scatter elastically, producing recoil protons, carbon and oxygen nuclei in the forward direction in the laboratory system. The dt neutrons can also undergo (n, p) or (n, α) reactions with carbon or oxygen nuclei, generating charged particles that can produce tracks on the front and/or back side of the CR-39. The CR-39 neutron detection efficiency is the probability that an incident neutron generates a charged particle, which leaves a visible − track on the etched CR-39 surface, and it is ∼10 4 for dd neutrons and ∼5×10−5 for dt neutrons. For more details about how the CR-39 is used as neutron detector for ICF applications, the reader is referred to [143, 144].

Figure 15. The CEA neutron imaging system on OMEGA-60. The 5.6. Recoil-particle systems penumbral aperture is centered at 26.6 cm from the implosion, and the detector can be positioned at either 4 or 13 m from the implosion 5.6.1. Magnet-based spectrometers. As discussed in section 2, fi [ ] for different magni cations. Reprinted from 138 , with the obtaining experimental information about Ti, Yn, ρR and their permission of AIP Publishing. asymmetries is central to building a picture of an implosion during the nuclear phase. To access this information, a neutron of broken molecular chains and free radicals. The level of spectrometer called a magnetic recoil spectrometer (MRS) has damage along the path is related to the rate at which energy is been implemented and extensively used for measurements of the lost by the charged particle (or dE/dX in the CR-39).The absolute neutron spectrum in the range of 5–30 MeV at second step involves making the trails of damage visible for OMEGA-60 and the NIF [145–149]. This range covers all detection. This is done by etching the CR-39 in NaOH, in which essential details of the ICF neutron spectrum shown in figure 1. the CR-39 surface is etched away at a bulk-etch rate (of In addition, the MRS nicely complements the nTOF − ∼2 μmh 1 in 80 °C6MNaOH), while damaged material along spectrometers used to diagnose the same implosion parameters. the particle trail etches at a faster rate called track-etch rate. As discussed in detail in [145, 146], the MRS consists of To determine the absolute primary-neutron yield from the either a CD (or CH) foil positioned close to the implosion for CR-39 data, the neutron-interaction processes and generation generation of recoil deuterons (or protons), a focusing magnet

19 Plasma Phys. Control. Fusion 62 (2020) 023001 Topical Review

Figure 16. The one-dimensional imager of neutrons (ODIN) on the Z-facility. The ray trace shown as yellow indicates the signal trajectories through the system. An exploded view of the slit assembly is shown in the insert. The primary and secondary detector locations are also shown. Reprinted from [139], with the permission of AIP Publishing.

Figure 17. The NIF neutron imaging system for simultaneously imaging primary and down-scattered neutrons. The system consists of an aperture, two collimators and a detector system. The insert in the top-left corner illustrates the array of pinholes and penumbral apertures used for the neutron-imaging system. Adapted with permission from [142].©(2017) Society of Photo-Optical Instrumentation Engineers (SPIE).

located outside the target chamber for energy dispersion and focusing of the recoil particles onto the focal plane of the spectrometer, and an array of CR-39 detectors positioned at the focal plane. The foil thickness and area are of order 100 μm and 10 cm2, respectively. An aperture of ∼20 cm2 is positioned in front of the magnet for the selection of forward- scattered recoil particles, and the array of CR-39 detectors records the position of each recoil particle with a detection efficiency of 100%. A schematic of the MRS on OMEGA-60 and the NIF is shown in figure 19. The efficiency (εMRS) of the system is given by Figure 18. An example of (a) a primary-neutron image, and (b) a Wf qmax dsq() down-scattered neutron image obtained from NIF shot N120205. eqMRS = ntdfò sindq .() 5.7 The size of the primary-neutron image is 28±3 μm, and the down- 2 0 dWlab ± μ scattered neutron image is 44 5 m. Qualitatively, the size Ω difference between these images provides information about the Here, f is the solid angle subtended by the foil, nd is the configuration of the high-density fuel shell that surrounds the deuterium (or hydrogen) number density in the foil, tf is the hot-spot. thickness of the foil, and dσ(θ)/dΩlab is the differential cross

20 Plasma Phys. Control. Fusion 62 (2020) 023001 Topical Review that the shielding reduces the neutron fluence about two orders of magnitude at the NIF. Additional reduction of the neutron-induced and intrinsic background is required for the down-scattered neutron measurement [151]. This is accom- plished by the coincidence counting technique (CCT), which utilizes the fact that incident signal particles (protons or deuterons) pass straight through the CR-39 material, resulting in front and backside tracks that are correlated, while neutron- induced and intrinsic background tracks are mainly on one of the surfaces. Applying the CCT to OMEGA-MRS data has demonstrated orders of magnitude S/B improvement [151]. Several options are available for configuring the MRS: the foil composition determines whether recoil protons or Figure 19. A schematic of the MRS. The main components of the MRS are a CD (or CH) foil, a magnet, and an array of CR-39 deuterons are used, and the foil area and thickness dictate the detectors. The foil is positioned 10 and 26 cm from the implosion at energy resolution and detection efficiency. As for other OMEGA and the NIF, respectively; the magnet is positioned 215 cm neutron diagnostics, the MRS performance improves as yield from the foil on OMEGA and 570 cm from the foil on the NIF. An and ρR increases (under the assumption that the signal and × 2 array of eleven and nine 7 5cm CR-39 detectors are positioned background levels do not saturate the diagnostic). Another at the OMEGA-MRS and NIF-MRS focal plane that is 166 and 84 cm long, respectively. The trajectories shown are for proton system, based on the MRS concept has also been designed in energies from 6 to 30 MeV, corresponding to deuteron energies from China [152]. 3 to 15 MeV. Reprinted from [146], with the permission of AIP The neutron spectrum is determined from a forward-fit, Publishing. convolved with the IRF, to the measured position histogram of the recoil protons (or deuterons). An important strength of section for the elastic scattering in the laboratory system, the MRS is that the technique is accurately characterized from which is integrated over the scattering angles defined by the first principles (ab-initio), enabling quantitative IRF calcula- − aperture. Typical efficiencies are in the range of 10 9 to tions to be performed before the system has been built. An in- − 10 12, depending on configuration. The total energy resolu- situ calibration was required, however, to check that the tion (ΔEMRS) is part of the spectrometer IRF, which is defined systems had been built and installed according to specifica- as the recoil-particle spectrum at the detector plane when tion. An example of MRS data from the NIF is shown in viewing a fluence of mono-energetic neutrons, and can be figure 21. expressed as

222 D=D+D+DEEEEMRS foil kin magnet ,5.8()6. Gamma-ray diagnostics Δ where Efoil is the broadening associated with the energy- In contrast to the many types of neutron diagnostics used at an loss distribution of the recoil particles in the foil (depending ICF facility, the number of gamma-ray techniques used to on the scattering location in the foil, the recoil particles show diagnose an implosion is quite limited. One of the reasons for an energy-loss distribution from zero up to a maximum this is that the gamma-ray yield per produced neutron is very ) Δ energy loss proportional to tf , Ekin is the kinematic-energy low [153] (see the branching ratios in equations (3.2), (3.3), broadening dictated by the scattering-angle distribution of the (3.6) and (3.11)), limiting the types of diagnostics that can be recoil particles (which depends on the position and area of the used for ICF applications. The advantage of using gamma- ) Δ foil and magnet aperture , and Emagnet ion-optical broad- rays to diagnose an ICF implosion, however, is that they do ening due to focusing aberrations in the magnet. These not display time-of-flight broadening at the diagnostic, which mechanisms are to the first order uncorrelated and Gaussian in enables gamma-ray-based systems to be positioned at rela- shape. The MRS system is optimized to maximize the tively large distances from an implosion in well-shielded fi ε fi Δ detection ef ciency MRS for a speci ed EMRS. locations. This makes gamma-ray diagnostics ideal for mea- The main source of background are neutrons scattered by surements of the nuclear burn history in environments with the chamber wall and other structures nearby the MRS. x-rays large x-ray and neutron backgrounds. As discussed in the next and gamma-rays are not an issue since the CR-39 is section, gamma-ray diagnostics also represent an excellent insensitive to these types of radiation. Even though the CR- complement to the NTD, PTD and PXTD as discussed in 39 efficiency for detecting primary neutrons is small, the section 5.3. impact of the neutron background must be reduced significantly for the down-scattered neutron measurement at 6.1. Gas-Cherenkov detectors OMEGA and the NIF. This is achieved by fully enclosing the spectrometer with polyethylene shielding as a first step (see Diagnostic systems based on the Cherenkov technique have figure 20), and positioning the CR-39 detector array in the been implemented and extensively used at high-yield facil- shadow of the NIF-target chamber. Through neutron-transport ities such as OMEGA-60 and the NIF [154, 155]. In these simulations using the MCNP code [150], it was established systems, the gamma-rays interact with a high-Z converter,

21 Plasma Phys. Control. Fusion 62 (2020) 023001 Topical Review

Figure 20. (a) The OMEGA-MRS fully enclosed by polyethylene shielding. (b) The NIF-MRS fully enclosed by polyethylene shielding. (c) Cut-out view of the NIF-MRS illustrating the spectrometer inside the shielding. Reprinted from [146], with the permission of AIP Publishing.

Figure 21. (a) MRS spectrum from NIF shot N150218. A CD foil with thickness and area of 55.8 μm and 3 cm2 was used, respectively. (b) Modeled neutron spectrum that provides the best fit to measured recoil-deuteron spectrum. From the modeled neutron spectrum, Yunsc, dsr (or ρR) and Ti were determined. positioned at the diagnostic front end, which generates for- Two types of Cherenkov-based diagnostics have been ward-directed relativistic electrons (mainly through the implemented by LANL and collaborators on OMEGA-60 and Compton-scattering process) that are emitted into a high- the NIF. The first system is the Gamma Reaction History pressure gas cell. If the electrons travel faster than the speed (GRH) system [154, 155], which is positioned on the of light in the gas, typically CO2 gas, the electrons generate OMEGA-60 and NIF target chamber at approximately 1.7 m Cherenkov light in the UV/visible range. The minimum and 6 m from implosion, respectively. The GRH system on gamma-ray energy (Eγ) required for generation of Cherenkov the NIF consists of four high-pressure gas cells, each pres- [ ] light is described by 156 surized to a specific pressure to probe gamma-rays above a ⎡ ⎤ certain energy (see figure 22). With four channels operated ⎢ 1 1 ⎥ 2 Eg = - mce ,6.1()with different energy thresholds, typically at 2.9, 5.0, 8.0, and ⎣⎢ 2 2 ⎦⎥ 11- /n 10 MeV, this diagnostic probes the emission history of 16.75 MeV gamma-rays produced by dt reactions and where n is the refractive index of the medium, me is the rest 12 mass of the electron, and c is the speed of light in vacuum. 4.44 MeV γ-rays generated by C(n, nγ) reactions in the This expression is derived under the consideration that the full remaining HDC or CH ablator [157, 158]. The NIF-GRH uses backscatter (i.e. 180° Compton-scattering) sets the lower a PMT, resulting in an IRF of 86 ps, which prevents mea- energy limit. surements of high-order moments in the gamma-ray emission The generated Cherenkov light is relayed to either an history. optical streak camera or PMT, using a series of off-axis As higher-order moments in the nuclear burn history parabolic mirrors. The voltage signal from the PMT is used to contain important information about shock reverberations and modulate a light signal, via a Mach–Zehnder interferometer, burn truncation (due to various implosion failure modes),a which is relayed with high fidelity to a digitizer. faster gamma cherenkov detector (GCD) shown in figure 23

22 Plasma Phys. Control. Fusion 62 (2020) 023001 Topical Review

Figure 22. (a) Picture of the 4-channel gamma reaction history (GRH) diagnostic on the NIF for measurement of the gamma-ray emission history. (b) Schematic of one GRH channel. This configuration generates a time delay of 4.3 ns that allows the detector to recover from prompt radiation due to laser-plasma interactions from the target. See text for more details.

Figure 23. (a) Picture of the 1-channel gamma cherenkov detector (GCD) for measurement of the gamma-ray emission history. (b) Schematic of the GCD design and light-collection scheme. was implemented on both OMEGA-60 and the NIF by LANL and collaborators [159]. The GCD is based on the same principle as GRH but with a different geometry and light collection scheme. The system can also be inserted inside the target chamber for maximum efficiency and measure gamma- ray emission at low yield levels [160, 161]. An example of GCD data obtained at OMEGA-60 is shown in figure 24. The latest incarnation of this system, currently known as Super GCD [162], has an energy threshold of 1.8 MeV and high sensitivity. As discussed in section 9, a GCD combined with a pulse-dilation-drift tube (PDDT) for significantly improved time response represents the next-generation system. Other media for the generation of Cherenkov light are also being explored [163]. Figure 24. An example dt gamma-ray emission history measured with the GCD system on OMEGA (shot 55983). The GCD was operated with a CO2 gas at 100 psi, which sets the threshold for generating Cherenkov light at an energy of 6.3 MeV. The back- 7. Charged-particle diagnostics ground signal was obtained on another shot (shot 55989) when the GCD was operated with no gas. In contrast to neutrons, charged particles interact electro- magnetically with the fuel, ablator and fields, and thus carry inherently more information about an ICF implosion than by Séguin et al [164], yields and energy spectra of these neutrons. As shown by equations (3.1)–(3.23), a wide variety charged particles are directly related to the properties of an of discrete lines and continua made up by protons, deuterons, ICF implosion, and specifically carry information about tritons and alphas are generated, where the discrete lines are fusion yields, Ti, convergence, ρR, Te, nuclear-burn history, generally produced by fusion reactions and continua are and mix (in some specific cases). In this section, the charged- caused by elastic scattering, inelastic scattering, and in-flight particle diagnostics used to routinely diagnose an ICF nuclear reactions. As discussed in section 4 and in the paper implosion are discussed.

23 Plasma Phys. Control. Fusion 62 (2020) 023001 Topical Review

Figure 25. (a) Microscope image of proton tracks with diameters in the range of 5–10 mm, which were revealed by etching the CR-39 for 6 h at 80 °C in 6.0-Molar NaOH. These tracks were generated by protons with energies in the range of 2–4 MeV. (b) Typical proton-track diameter as function of energy for CR-39 etched for 6 h at 80 °C in 6.0-Molar NaOH. It should be noted that some variations in the diameter- versus-energy curve are often observed, depending on the quality of the CR-39.

7.1. CR-39 detectors While neutrons interact volumetrically with the CR-39 mat- erial (see section 5.5), energetic charged particles start to interact immediately with the CR-39, resulting in a nanometer sized trail of damage at the front surface that depends on dE/ dX. As discussed briefly in section 5.5, these trails are revealed by etching the CR-39 in NaOH at certain temper- ature and concentration (80 °C 6-Molar NaOH is commonly used). If the charged particle enters the CR-39 surface with perpendicular incidence, the etched track is round and the diameter provides information about energy and particle specie. Figure 25(a) shows an image of proton tracks with diameters in the range of 5–10 mm, and figure 25(b) shows the Figure 26. A schematic of the WRF spectrometer, which consists of track diameter as a function of proton energy when the CR-39 a wedge filter on top of a piece of CR-39. Due to its compactness was etched in 6.0-Molar NaOH at 80 °C for 6 h. (∼5 cm in diameter), several spectrometers can be fielded around the implosion at close distances. See text for more details. 7.2. Wedge-range-filter (WRF) spectrometers energy cutoff is ∼20 MeV for protons. As the minimum and WRF spectrometers are compact systems that consist of a piece maximum energies at each location behind the wedge filter fi [ ] ( of CR-39 positioned behind a wedge-shaped lter 164 see are known by design and the fact that they correspond to the fi ) fi ( gure 26 . The wedge lter, typically made of aluminum but largest and smallest track diameters, respectively, only ) zirconia ceramic has also been used , disperses charged parti- information about the relative shape of the CR-39 response cles onto the CR-39 based on the particle-energy ranging in the curve is required to interpolate between these points (a typical fi fi lter. The combination of the wedge lter and CR-39 limits the curve is illustrated in figure 25(b)). This characteristic makes minimum and maximum detectable charged-particle energies at the WRF spectrometer relatively immune to piece-to-piece a certain location on the CR-39. The reason for this is that variations in the absolute CR-39 response to charged parti- charged particles must be energetic enough to penetrate the cles. Figure 27 illustrates the WRF spectrometers fielded wedge filter but not too energetic not to be registered on the 13 inside the OMEGA and the NIF target chambers. CR-39 . This means that at each location along the disper- WRF spectrometers have been used extensively for sion direction, there exists a narrow energy range over which directional measurements of primary D3He-protons for stu- charged particles are detected with 100% detection efficiency. dies of ρR asymmetries, fuel-shell mix, implosion dynamics, From the diameter of a track, combined with the information hydrodynamic equivalence and ablator burn-through in D3He about the local wedge-filter thickness, the energy of the gas-filled implosions [165–176]; for measurements of sec- incoming charged particle is determined. The low-energy 3 ρ cutoff for proton measurement is ∼4 MeV, while the high- ondary D He-protons for studies of R asymmetries in D2 gas-filled warm, shock-ignition, fast-ignition, and cryogeni- cally-layered implosions [79, 176–186]; and for measure- 13 / If a charged particle have too high energy, the dE dX is not high enough fi to create enough damage in the CR-39 to generate a track that can be made ments of knock-on protons produced in DT gas- lled visible with the NaOH etching process. implosions [187–189]. Given the compactness of the WRF

24 Plasma Phys. Control. Fusion 62 (2020) 023001 Topical Review

Figure 27. (a) Seven WRF spectrometers (six marked blue and one green) and a magnetic spectrometer (magenta) fielded on OMEGA. (b) Four WRF spectrometers fielded on the pole and equator on the NIF. The WRF spectrometers are mounted on the DIMs using external mounting brackets. (c) Picture of a NIF shot on which the WRF spectrometers were used on the pole and equator.

Figure 28. Primary d3He-proton spectra measured with WRF spectrometers positioned at various locations around an OMEGA implosion (shot 21240). A 15 atm D3He gas-filled CH capsule with a wall thickness of 20 μm, and sixty laser beams delivering 12 kJ in a 1 ns square pulse to the capsule were used in this experiment. The WRF spectrometers were inserted by the ten-inch manipulators (TIM, six indicated in the figure). Both the average d3He-proton birth energy (vertical dashed line) and average energy loss measured in the different directions are indicated. The variation in the observed energy loss is substantial, indicating significant ρR asymmetries. spectrometers, they are routinely fielded in-close and around 106, respectively. These systems have been essential to both the implosion to provide information about yield and ρR basic physics studies and to the main ICF programs at asymmetries for primary and secondary yields above 105 and OMEGA and the NIF. A few examples of WRF-measured

25 Plasma Phys. Control. Fusion 62 (2020) 023001 Topical Review

Figure 29. Secondary d3He-proton spectra measured with WRF spectrometers positioned at various locations around an OMEGA cryogenic implosion (shot 24096). A 100 μmD2 layer inside a 930 μm diameter GDP capsule, illuminated by sixty laser beams delivering 24 kJ in a 1.0 ns square pulse were used in this experiment. The average secondary d3He-proton yield was 2.4×107 and the primary dd-neutron yield was 3.1×1010. Both the average secondary d3He-proton birth energy (vertical dashed line) and average energy loss measured in the different directions are indicated. spectra are shown in figures 28–31. In addition, similar WRF (classical) spectrometers are now in operation at the SG-III laser in 1 China [190, 191]. r = 2,mEii () 7.1 ZeB Although WRFs spectrometers are well-suited for mea- i surements of protons, they cannot be used accurately for where Zi is the ion-charge number, e is the elementary charge, spectral measurements of heavier ions for a couple of reasons. mi is the ion mass, and Ei is the ion kinetic energy. The The WRFs rely on charged-particle track diameter and con- magnetic Charged-Particle Spectrometers (CPS1 and CPS2), trast on the CR-39 for the discrimination of ion species. implemented on OMEGA, utilize a 7.6 kGauss permanent Though the differences in contrast and diameter are measur- magnet and CR-39 detectors for momentum-dispersion and able between protons and heavier ions, they are subtle among detection of different types of ions, respectively (see heavy ions. Thus, it is difficult, if not impossible, to discern figure 32). As these systems are not focusing devices, a different heavy-ion species using this approach. In addition, narrow and interchangeable vertical slit (width in the range of the thin end of the wedge readily stops energetic heavy ions, 0.1–2.0 mm) is used as an aperture to provide the required thereby significantly increasing the low-energy limit for this energy resolution and efficiency for different applications. measurement. These spectrometers are used routinely at OMEGA for measurements of primary and secondary charged fusion 7.3. Magnetic spectrometers products. They also complement the WRF spectrometers in their ability to measure spectra down to a proton-equivalent 7.3.1. The charged particle spectrometer. Magnetic energy of 100 keV for yields above 108. From equation (7.1), spectrometers have been implemented for ICF applications it is clear that degeneracy exists between different ions at both OMEGA and the NIF, and these spectrometers use a species. For example, a 1.1 MeV triton will have the same permanent magnet to separate ions based on their gyro radius gyro radius as a 3.3 MeV proton and 3.3 MeV alpha, and

26 Plasma Phys. Control. Fusion 62 (2020) 023001 Topical Review

Figure 30. Knock-on proton spectra measured with WRF spectrometers positioned at different locations around an OMEGA implosion (shot 23471). A 15 atm DTH gas-filled (5% D, 5% T and 90% H) CD capsule with a wall thickness of 20 μm, and sixty laser beams delivering 24 kJ in a 1.0 ns square pulse to the capsule were used in this experiment. The average knock-on proton yield (per MeV) was 1.9×107 and 11 −2 the primary dt-neutron yield was 1.6×10 .AρRfuel of 6.4 mg cm was inferred from the average knock-on proton yield (in the range of −2 9.5–10.5 MeV), and from the average energy loss of the maximum knock-on proton energy (i.e. 14 MeV),aρRshell of 70 mg cm was −2 determined. The spectra observed in the different directions display a ρRshell asymmetry of 25 mg cm . The knock-on deuteron component from the CD shell is evident at proton-equivalent energies below ∼9 MeV. Reprinted from [187], with the permission of AIP Publishing. hence end up at the same location along the dispersion plane. respectively [192]. This diagnostic, which is an extension of However, these ions can be separated based on track diameter the pTOF diagnostic discussed in section 5.2.2, utilizes a and contrast, as illustrated figure 33. Examples of CPS- bending magnet combined with a shielded CVD detector. It measured spectra are shown in figures 34 and 35. can also be used in spectrometry mode by replacing the CVD fi CPS1 and CPS2 cannot be used for spectral measure- with a CR-39 detector. The diagnostic, shown in gure 36,is ments of heavy ions (Z>2). The reason for this is that nominally positioned 50 cm from the implosion using a DIM. discrimination of heavy-ion species is difficult because the The primary motivation for magPTOF is to reduce the fl fi variation of track diameter and contrast are weak among large x-ray ux generated in a gas- lled hohlraum implosion, 3 heavy ions with different energies and charge states. On the which typically overwhelms the shock-generated d He-proton other hand, spectral measurements of low-Z ions in the signal. The inclusion of the thick x-ray filter in front of presence of heavy ions are possible because filters (made of the CVD detector and a deflecting magnet increases the either Mylar, aluminum or tantalum), thick enough to stop the d3He-proton signal to x-ray background about 1000×. heavy ions (e.g. carbon), are positioned in front of the CR-39 The d3He protons are deflected around the x-ray filter by detectors. the magnet, while the x-rays are greatly attenuated through it. An example of magPTOF data is shown in figure 37 for NIF 7.3.2. The magnetic particle-time-of-flight (magPTOF) shot N151221. As shown by the signal trace, the data is of diagnostic. On the NIF, the magPTOF diagnostic has been high quality and indicates a shock-bang time of 6.7±0.2 ns implemented primarily for simultaneous measurements of and a compression-bang time of 7.3±0.1 ns (when the flight shock-bang time and compression-bang time in D3He gas- time and IRF have been deconvolved from the measured filled implosions, using d3He-protons and dd-neutrons, signal trace). This differential measurement of the shock-bang

27 Plasma Phys. Control. Fusion 62 (2020) 023001 Topical Review deflection regions. These expressions are valid for small deflections. The TP system thus enables simultaneous spectral measurements of a number of heavy-ions species. For this reason, two TP systems were implemented on OMEGA [193, 194]. One system, called Thomson Parabola Ion Spectrometer (TPIS) [193], was built to probe heavy ions. This system, shown in figure 38, comprises of a 400 μm diameter tantalum aperture positioned about 50 cm from the experiment, a per- manent magnet, electrostatic deflector plates, and a detector assembly consisting of a piece of CR-39 and an image plate (IP). The aperture defines the energy resolution and field of view (and thus efficiency). The magnet has a 1 cm pole gap 2 Figure 31. An example of a WRF-measured primary d3He-proton and 5.1×5.1 cm pole surfaces, resulting in a uniform spectrum from NIF implosion N091123. For this experiment, a magnetic field of 5.3 kG. The electrostatic deflector plates are μ fi 3 135 m thick CH capsule lled with 10% D2 and 90% He was 2 cm apart, and these plates can be operated with a voltage up used. The spectrum displays two components, the first is a shock 2 3 to 80 kV. 10×10 cm sized CR-39 and a Fuji-TR IP, or a component where d He protons are generated shortly after the shock fi rebounds at the center of the implosion when the total ρR is stacked assembly of both can be elded in the diagnostic. To relatively low. The second component is due to d3He protons facilitate absolute measurements of proton spectra, the Fuji- produced during the compression phase, about 400 ps after the shock TR-IP response to protons was determined for energies in the phase, when the total ρR is substantially higher. Thus, the time- range of 1–8 MeV [193]. In addition, as the Fuji-TR IP does integrated spectrum is a mainly a reflection of the ρR time evolution not have a protective layer of Mylar normally found on other (other effects such as Doppler broadening and geometrical energy- loss effects play a role when the ρR is relatively low). IPs, carbon ions can also be detected while heavier ions are stopped before they can deposit energy in the active IP layer. and compression-bang times provides valuable additional The Fuji-TR IP is therefore ideal for measurements of ener- constraints on the 1D physics of the shock propagation and getic protons while CR-39 is more important for detecting shell deceleration, improving the understanding of the heavier ions. in-flight conditions of the fuel and shell. Another TP system is the Thomson Parabola Ion Energy (TPIE) analyzer [194]. This system was originally designed 7.4. Thomson parabolas (TP) for studies of fast ions generated by short-pulse laser-plasma interactions at OMEGA-EP, but it has also been used for A TP utilizes the combination of electric and magnetic fields measurements of fast ions generated in ICF implosions at and has the advantage over magnetic spectrometers because it OMEGA. The TPIE system, shown in figure 39, uses an breaks the charge-to-mass Zie/mi degeneracy associated with interchangeable square tungsten aperture with a diameter of magnetic dispersion. On the other hand, as TP requires a 100 μm, 250 μm or 1 mm, an interchangeable permanent small pinhole as aperture to provide acceptable energy reso- magnet with 10×5cm2 pole surfaces (and with either 5.6 or lution, the efficiency is very low in comparison to the effi- 8.4 kG peak field strengths), a pair of 21 cm long parallel ciency provided by magnetic spectrometers. electrodes with a 1 cm separation gap that can generate a A TP system utilizes a parallel (or anti-parallel) voltage of 20 kV (total), and a detector. The detector pocket arrangement of electric and magnetic fields, which generates accommodates a 10×5cm2 piece of CR-39, an IP, or an discrete signal parabolas on the detector plane with constant assembly of both. An IP is needed for detection of high- Zie/mi ratios. Each position along one of these parabolas energy (>10 MeV) protons, while CR-39 is needed for corresponds to a unique energy of the detected ion. In the case detection of heavy ions. As the TPIE is fielded by a TIM, the of parallel homogeneous E and B fields, the position of the distance from the aperture to the experiment can be varied / charged particles with a given Zie mi at the detector plane is from 37 to 150 cm. In contrast to the TPIS, this flexibility, given by combined with the interchangeable aperture, allows for a ZeE trade-off between the energy resolution and efficiency. x »+i LD()0.5 L , () 7.2 2 11 1 Examples of TPIE data are shown in figure 40. mvi i and

ZeBi 7.5. Nuclear burn-history detectors y »+LD22()0.5 L 2 , () 7.3 mvii Both the PTD and PXTD have been used routinely for where e is fundamental charge, mi is the ion mass, E and B are measurements of the emission history of charged fusion the electric and magnetic field strengths, respectively; L1 and products. As these diagnostics are discussed in detail in L2 are the lengths of E-field and B-field deflection, respec- section 5.3 and by Sio et al [134], further elaboration will not tively; and D1 and D2 are the detector distances to the be made here.

28 Plasma Phys. Control. Fusion 62 (2020) 023001 Topical Review

Figure 32. (a) Schematic of the charged-particle spectrometer (CPS). (b) Picture of CPS1 system (new location) on the OMEGA chamber. The magnet is inside the cylindrical vacuum chamber. (c) Picture of the shielded CPS2 system during installation. The magnet is located in front of the assembly. Both spectrometers are permanently mounted on fixed diagnostic ports on the OMEGA target chamber. The CPS1 and CPS2, which are positioned 100 cm and 235 cm from an implosion, respectively, provide close-to orthogonal views of it, thereby allowing for an assessment of yield and ρR asymmetries. Reprinted from [164], with the permission of AIP Publishing.

Figure 33. Microscope image of 1.1 MeV triton tracks, 3.3 MeV Figure 34. dd-triton, d3He-alpha, dd-proton, and d3He-proton spectra proton tracks, and 3.3 MeV alpha tracks recorded by the CPS2 measured simultaneously with CPS2 for OMEGA shot 75699. These system. These ions were generated by dd and d3He reactions in a 3 fusion products are produced by the reactions shown in D He gas-filled, thin-glass implosion (OMEGA shot 20297). ( ) ( ) [ ] equations 3.5 and 3.10 . The vertical arrows and dashed lines Reprinted from 164 , with the permission of AIP Publishing. indicate measured median energy and average birth energy, respectively. Reprinted figure with permission from [73], Copyright 7.6. Imaging systems 2019 by the American Physical Society. As discussed in section 5.4, significant efforts have been made over the last couple of decades to image dt-burn profiles aperture is larger than the dd- and d3He-burn profiles, all using 14 MeV neutrons. During this time period, a com- information about the burn region is encoded in the penumbra plementary approach has been developed and used to image of the image, which must be post-processed with special dd- and d3He-burn profiles using 3.0 MeV protons and inversion algorithms that reconstruct the burn distributions. 14.7 MeV protons [196, 197], respectively. In contrast to the Up to three imaging systems can be fielded on OMEGA using neutron-imaging technique that uses a long and tapered pin- TIMs with nearly orthogonal view of the implosion, as illu- hole (and/or penumbral aperture), this charged-particle-ima- strated in figure 42. ging technique is performed with a penumbral imaging Figure 43 illustrates a set of images of the d3He-proton system that is based on a thin aperture. In addition to the thin surface-brightness observed in the three directions, as indi- aperture, which is significantly larger than the size of the dd- cated in figure 42(b), for an intentionally asymmetrically- and d3He-burn profiles, this system consists of CR-39 driven implosion with a symmetry axis around the view-1 detectors that are separated by ranging filters for detection of axis. For more in-depth discussion about this type of mea- the different charged fusion products (see figure 41). As the surement, the reader is referred to [86, 198, 199].

29 Plasma Phys. Control. Fusion 62 (2020) 023001 Topical Review

Figure 35. Knock-on deuteron (d′) and triton (t′) spectra measured simultaneously with CPS2 for three OMEGA shots. These spectra were generated by 14 MeV dt neutrons elastically scattered off fuel ions (equations (3.13) and (3.14)) in DT gas-filled, thin-glass implosions. The broadening of the spectra is mainly due to Doppler broadening and CPS-spectrometer response. Adapted figure with permission from [63], Copyright 2011 by the American Physical Society.

Figure 36. The magPTOF on the NIF. (a) A schematic of the magPTOF main components. The x-ray filter blocks the CVD detector from x-rays coming directly from the implosion, while the d3He-protons are deflected around it. (b) A CAD drawing of the magPTOF and WRF spectrometers attached to a snout inserted by a DIM.

7.7. Mono-energetic charged-particle radiography is typically used in combination with a CR-39 detector14. The backlighter is generally positioned ∼1 cm from the subject The use of quasi-mono-energetic charged particles, from and the CR-39 detector is positioned on the other side of the primary fusion reactions, to radiograph ICF-relevant plasmas subject ∼30 cm away, resulting in a magnification of ∼30. In has opened a new window to, and provided several profound addition, a mesh is used in some experiments, and this mesh insights about, the physics governing ICF-relevant plasmas is positioned in front of the detector package. A schematic of [ – ] 88, 89, 91, 92, 200 221 . In contrast to x-rays, which have the experimental configuration for a charged-particle-radio- been used for decades to radiograph plasma-density dis- graphy experiment is shown in figure 44. tributions, charged particles are sensitive to both matter and The backlighter consists typically of a thin-glass capsule electromagnetic fields, where the latter is the physics of main (outer diameter of ∼420 μm and wall thickness of ∼2 μm) interest in most radiography experiments. To interrogate the filled with equimolar D3He gas (total pressure of 18 atm). electromagnetic fields in an ICF-relevant plasma, a point- projection backlighter that produces primary fusion products 14 IPs are used in some experiments.

30 Plasma Phys. Control. Fusion 62 (2020) 023001 Topical Review

Figure 37. MagPTOF data for shot N151221. After the IRF and particle flight times have been corrected for, a shock-bang and compression-bang time of 6.7±0.2 ns and 7.3±0.1 ns were determined, respectively, indicating a time difference of 0.6 ns.

Figure 39. A cutaway view of TPIE, which includes a pinhole assembly, permanent magnet (blue), electrodes (yellow), and a CR- 39 detector. The diagnostic is fielded using a TIM. Reprinted from [194], with the permission of AIP Publishing.

Figure 38. A cutaway view of TPIS, which includes a pinhole aperture, permanent magnet (blue), electrodes (red), and detector pocket. The outer dimensions of the diagnostic are also indicated. Reprinted from [195], with the permission of AIP Publishing.

∼ Figure 40. An example of TPIE data obtained from two ICF This capsule is illuminated by 20 non-smoothed laser ( ) ∼ implosions at OMEGA. Top image TPIE data from a thin-glass beams, delivering 9 kJ to the capsule in a 1 ns square pulse. implosion (shot 64685), and (bottom image) from an aluminum This allows for the other 40 laser beams to be used for the flash-coated, CD-capsule implosion (shot 65273). The ion track actual experiment. The D3He gas in the backlighter is shock- density is represented by a color value. Several lines of highly ionized silicon and oxygen ions are observed from the thin-glass heated and compressed to Ti of ∼10 keV and number den- 23 −3 ∼ 9 ∼ 10 capsule implosion, and several lines of ionized aluminum and sities of order 10 cm , which generates 10 and 10 fl 3 oxygen ions are observed from the ash-coated CD-capsule d He and dd reactions, respectively, over a burn duration in implosion. These data were obtained by operating the TPIE with a the range if 100–150 ps and burn volume (source size) with a piece of CR-39 as detector and peak magnetic and electric fields of diameter of order 50 μm. 5.4 kG and 2.8 kV cm−1, respectively. As the Lorentz force causes charged-particle deflections, information about the path-integrated electromagnetic fields and in the subject plasma is contained in the charged-particle Zei spatial fluence variations at the detector plane. Given the qE = Edl^ ,7.5() 2E ò mono-energetic nature of the charged-particle backlighter, i there is a direct correlation between the deflection angle and where Zi is the ion-charge number, e is the elementary charge, path-integrated E and B fields as described by mi is the ion mass, and Ei is the ion kinetic energy. B⊥ and E⊥ are the magnetic and electric fields perpendicular to the Zei charged-particle trajectory. From these two equations, it is qB = ò Bdl^ ,7.4() 2mEii clear that the effect of both E and B fields can be determined

31 Plasma Phys. Control. Fusion 62 (2020) 023001 Topical Review

Figure 41. A schematic drawing of the charged-particle imaging system. With an aperture of ∼2000 μm that is significantly larger than the burn region, information about the source exists in the penumbra. The surface brightness of the charged-particle emission is reconstructed from the measured image shown to the right.

Figure 42. (a) A drawing of the charged-particle imaging system that is used to primarily measure the dd and d3He burn regions. The CR-39 detector (separated from the system in this figure) can be positioned at three different slots at different distances for different magnification. (b) Cutaway view of the OMEGA-target chamber, showing three imaging systems pointed toward the implosion (located at the center of the chamber). The blue structure holds the capsule. Adapted from [196], with the permission of AIP Publishing.

Figure 43. Three d3He-proton surface-brightness images observed in three nearly orthogonal directions. These images were obtained from a D3He gas-filled CH-capsule implosion (OMEGA shot 35172), which was asymmetrically driven (with a symmetry axis parallel with View 1). from radiographs of, for instance, 3 MeV dd-protons and understanding of how the finite source size, scattering in the 14.7 MeV d3He-protons. To extract this information from the plasma subject, and scattering in the detector impact the radiographs, a fundamental understanding of the spectral, radiograph must also be obtained. By design, the latter two temporal, and emission characteristics of the charged particles are insignificant and can be ignored, which means that the emitted from the backlighter must be obtained. An spatial resolution is limited by the charged-particle source

32 Plasma Phys. Control. Fusion 62 (2020) 023001 Topical Review

Figure 44. Schematic of the charged-particle radiography measurement. The backlighter generates quasi-mono-energetic charged particles that are used to radiograph a plasma subject. A package of CR-39 detectors, properly filtered for the different charged particles, is used for detection of these particles. Most experiments to date have utilized 3 MeV dd-protons and 14.7 MeV d3He-protons, in which two pieces of CR-39 were fielded, where each one was filtered to optimally detect these particles. As discussed in section 9.3, advances are being made to this platform, where three charged particles will be used simultaneously to radiograph a plasma subject. size. Examples of charged-particle radiographs are shown in directed to the first stage of RAGS where water vapor, figures. unwanted species and reactive gases are filtered out from the gas. This procedure generates a noble-gas fraction for addi- tional purification. The RAGS is typically set to collect xenon or a combination of xenon and krypton. This selection is done 8. Radiochemical diagnostics by adjusting the temperature in a cryo trap. Radioactive iso- topes are first analyzed using high-purity germanium gamma- Two types of radiochemical diagnostics are being used routinely ray spectrometer, and isotopes with half-lives as short as eight to support the ICF program at the NIF. These are the radio- seconds can be measured [225]. The gases are subsequently chemical analysis of gaseous samples (RAGS) system [222] and transferred to a removable sample bottle where they are fro- solid radiochemistry collector (SRC) [223, 224], which are both zen and analyzed with gamma-ray or noble-gas-mass used to measure nuclear reactions between neutrons or charged spectroscopy. To determine the total number of radioactive particles, emitted from an implosion, and a particular type of nuclei in the gas that were generated by the implosion, the nuclei implanted in either the capsule or hohlraum. After an RAGS efficiency for collecting reaction products from the implosion, the resultant radioactive gas or solid debris are col- target chamber must be determined. This is done in a three- lected from the NIF target chamber and analyzed via radiation step process. First, a small but known amount of either stable detection or mass spectrometry to determine the number of or radioactive tracer gas is injected into the NIF target reaction products produced in the implosion or hohlraum (mass chamber directly after a shot. Second, the gaseous products spectrometry is used to analyze non-radioactive isotopes). are pumped out and into the RAGS system. From the col- Essential to both techniques is to have a clear understanding of lected gas, the efficiency is determined by comparing the the spatial distribution of the implanted ions with respect to the number of injected tracer atoms to the number of radioactive source and flux distributions of the neutrons or charged particles. isotopes produced by the implosion. The collection and The reason for this is that the number of reaction products analysis of the gas is conducted with efficiencies in the range depends strongly on these distributions, as well as on the of 80%–100%. spectrum and reaction cross sections. An example of a RAGS application is when 127I and These radiochemical methods are being used to diagnose 124Xe are implanted into the capsule ablator. From 127I(d, the ratios of radioactive isotopes generated by charged par- 2n)127Xe and 124Xe(n,2n)123Xe reactions, the ratio of the ticles and neutrons at lower energies and 14 MeV, and from xenon isotopes in the collected gas provides information these ratios a spatially-averaged fuel ρR can be determined. about fuel-ablator mix. The challenge with this kind of measurement is to estimate the level of mix in an implosion. 8.1. System for RAGS The reason for this is that an absolute calibration of the col- The hardware for RAGS consists of a pre-filter that removes lection efficiency of the different isotopes is required. water vapor, particulates and reactive gases, and a cryogenic noble gas-collection system (see figure 45). The RAGS was 8.2. Solid radiochemistry collector designed to collect and analyze activated gases produced in the implosion. After an implosion, the activated gases in the The SRC diagnostic is used primarily to collect solid debris target chamber are pumped out by a series of turbo pumps and produced by an implosion. Two SRC designs were initially

33 Plasma Phys. Control. Fusion 62 (2020) 023001 Topical Review

Figure 45. The RAGS hardware at the NIF. Directly after a shot, the gaseous products inside the target chamber are pumped out and into RAGS, where unwanted species are filtered out from the gas (this happen in the right cart shown in the figure). The noble gases are subsequently frozen and analyzed to quantify the number of nuclear reactions that occurred in an implosion. This happens in the left cart shown in the figure. Reprinted from [222], with the permission of AIP Publishing.

Figure 46. (a) Pictures of the SRC systems. (a) The SRC with a 2 inch diameter disc of either tantalum, vanadium or carbon. Up to four SRC systems can be fielded on one DIM. Reprinted from [224], with the permission of AIP Publishing. (b) The Vast Area Detector for Experimental Radiochemistry (VADER) system for solid radiochemistry collection. implemented. The first design consists of a 2 inch diameter spectra in (figures 1 and 21(b))), different gold isotopes are disc of either tantalum, vanadium or carbon. This disc is generated by the 197Au(n, n′γ)198Au and 197Au(n,2n)196Au mounted in the same holder as used for a WRF spectrometer, reactions. As the (n, n′γ) cross section dominates the (n,2n) discussed in section 7.2, and up to four of them can be cross section at lower energies, and vice-versa at 14 MeV, the mounted on a DIM about 50 cm from an implosion ratio of the two gold isotopes provides relative information of (figure 46(a)) [226]. The second design consists of a large- the spatially-averaged fuel ρR in an implosion. For an abso- area detector, called Vast Area Detector for Experimental lute determination of the ρR, the SRC-gold data is cross- Radiochemistry (VADER) [223] (figure 46(b)), and this referenced to the dsr data obtained by the down-scattered- design has a six times larger solid angle than the standard neutron method (see figure 47). 2 inch-disc design. After an implosion, the SRC discs or VADER are removed for analysis using gamma-ray spectroscopy. Depending on the application, the SRC samples 9. Next-generation nuclear diagnostics may be first processed chemically to separate the material of interest from interfering background products. Since the initiation of the experimental ICF program in the The SRC system has routinely been used to collect 70s, substantial progress has been made, facilitated in part by activated gold debris from the holhraum. When the gold is the innovation and development of novel nuclear diagnostics exposed to the ICF neutron spectrum at the NIF (see typical that provided new and unprecedented measurements. Higher

34 Plasma Phys. Control. Fusion 62 (2020) 023001 Topical Review analyzed by the gamma-ray spectrometer. The times of arrival and pulse-height are recorded by a digitizer. Subse- quently, data-analysis routines are used to determine the characteristic gamma-ray events from the 89Zr. As the rate of 89Zr events is decaying with a 3.2 d half-life, the total number of neutrons incident on that detector from a particular shot is determined by extrapolating back to the time of the shot. For more detailed information about the RT-NADs, the reader is referred to [227, 228].

9.1.2. Cherenkov-based nTOF spectrometers. As discussed in section 5.2, nTOF spectrometers are being used to routinely measure the 0th moment (yield), 1st moment (hot-spot velocity), and 2nd moment (spectral broadening) of the dt- neutron spectrum. This has been done by recording neutron arrival times along different LOS using a scintillator coupled Figure 47. 198Au/196Au ratio determined from gamma-ray to a PMT. Recently, it has become clear that more precise spectroscopy of the resultant debris collected by the SRC on several measurements of the different spectral moments, than DT cryogenic implosions at the NIF (each data point represents one provided by standard nTOFs, are required. As typical hot- 198 197 198 implosion). The Au is produced by Au(n, γ) Au induced by spot flow velocities in NIF implosions are ∼50 km s−1, low-energy neutrons, which are generated single and multiple scatter measurement of the 1st moment in dt-neutron spectrum must in the fuel, and the 196Au is produced by 197Au(n,2n)196Au reactions that are induced by 14 MeV neutrons. Reproduced from [118]. be conducted to an accuracy much better than 0.1%. With an © IOP Publishing Ltd. All rights reserved. nTOF spectrometer positioned about 20 m from the implosion, this translates to a temporal precision of much better than 30 ps. To achieve this, the timing of the nTOF fusion yields have also enabled higher-quality measurements signal trace relative to the neutron production time must be with improved spatial, temporal or spectral information. The extremely well known (equation (5.4)). The IRF also needs be ability to gather more data in a single implosion experiment narrow and known accurately. has also facilitated an increased level of understanding about A new and exciting effort is under way within the ICF the physics governing the nuclear phase of an ICF implosion. program in the US to implement Cherenkov-based nTOF With the implementation of high-fusion-yield facilities world- spectrometers that will meet these requirements [229–231]. wide, such as the NIF, LMJ, UFL-2M and SG-IV facilities, The technique is based on Cherenkov radiation generated by the next-generation nuclear diagnostics will play an even (n, n′γ) reactions in fused silica followed by Compton scatter more important role for decades to come. In this section, we and pair production. Several prototype systems have been discuss today’s perspective on them. built and implemented recently on the NIF and OMEGA, and the first set of tests look very promising. The nuclear excitations and subsequent Cherenkov radiation generate a 9.1. Neutron diagnostics signal that is significantly shorter than the decay time of scintillator decay times. As the fused silica is sensitive to 9.1.1. Real-time neutron activation diagnostics. As discussed implosion (and hohlraum) gamma-rays, which are not in section 5.1.2, the zirconium NADs positioned around an affected by Doppler shifts, an accurate gamma-ray measure- implosion at the NIF are used to measure the directional yield ment of t can also be made simultaneously with the neutron of unscattered neutrons emitted from a high-ρR implosion. measurement. Concerns about the detection efficiency still From this type of data, a fluence (and ρR) map with remain to be addressed for certain applications. asymmetry modes up to L=2 can be reconstructed. However, the number of NADs has been insufficient to characterize asymmetries with modes larger than L=2. To 9.1.3. Three-dimensional neutron imaging. Diagnosing the address this issue, 48 real-time nuclear activation diagnostics 3D morphology of the nuclear burn and surrounding high- (RT-NADs) are currently being implemented at various density fuel with high spatial resolution is essential to locations on the NIF chamber. A key-feature of this system constructing an adequate picture of the assembled hot-spot is that it operates without depending on manual retrieval and and surrounding high-density fuel-shell. To that end, a 3D off-line analysis of the RT-NAD activation. neutron-imaging system for imaging both primary and down- Each RT-NAD consists of a zirconium disc, with a mass scattered neutrons is currently being implemented at the NIF. of about 400 g, a lanthanum-bromide-scintillator crystal The system will view the implosion along three close-to coupled to a photomultiplier, and a compact gamma-ray orthogonal LOS [232], which simplifies the reconstruction of spectrometer, which is used to record 909 keV gamma-rays an asymmetric implosion. Similar to the existing systems, from the activated 89Zr. The light generated in the crystal is these neutron-imaging systems consist of an array of pinholes converted to an electronic signal that is amplified and and penumbral apertures, a scintillator-based detector with

35 Plasma Phys. Control. Fusion 62 (2020) 023001 Topical Review

Figure 48. (Left) Schematic of the experimental setup for radiographing a cone-in shell implosion. For this measurement, a quasi-isotropic emission of ∼3×108 d3He-protons from the backlighter was used. These protons had a spectral linewidth of less than 3%, and they were produced over a time duration of 130 ps. (Right) Fluence radiographs of an unimploded cone-in shell capsule (shot 46531) and an imploded cone-in shell capsule (shot 46529). The dark areas in the radiographs correspond to regions of higher proton fluence. From these radiographs, details of the path-integrated field distributions surrounding the capsule can be determined. From [202]. Adapted with permission from AAAS. gated cameras to collect two timed images, one for 14 MeV with standard acquisition systems. Shielding will surround the neutrons and one for down-scattered neutrons. detector system for suppression of the ambient background of Using three orthogonal images of both primary and neutrons and gamma-rays [239]. down-scattered neutrons, analysis methods are currently being developed to generate a 3D picture of the burn region, surrounding high-density fuel-shell, and remaining ablator 9.2. Gamma-ray diagnostics [233–235]. 9.2.1. Three-dimensional gamma-ray imaging. The 3D neutron imaging system on the NIF is also being developed 9.1.4. Time-resolving neutron spectrometry. Current neutron with 4.44 MeV gamma-ray imaging in mind [234, 240]. With spectrometers are used routinely to measure the time-integrated an additional time gate, imaging of gamma-rays can also be ρ neutron spectrum, from which burn-averaged values of R, Yn, done. This data would provide 3D information about the and apparent Ti are determined, as discussed in sections 5.2 and location of the CH (or HDC) ablator and ablator mix into the 5.6. Although these data have been essential in guiding our dt fuel. understanding of the physics governing the nuclear phase of an ICF implosion, the current suite of spectrometers does not provide any information about the evolution of the fuel 9.2.2. Gamma-ray emission-history diagnostic. The current assembly, hot-spot formation, alpha heating, and nuclear burn. GCD system has been used at both OMEGA and the NIF for This information will be obtained with the next-generation measurements of the nuclear burn history using dt gamma- MRS for time-resolved measurements of the neutron spectrum. rays (equation (3.2)) or the ablator-ρR evolution using The plan with this system is to measure the spectrum in the 4.44 MeV gamma-rays from the 12C(n, nγ) reaction range of 12–16 MeV with high accuracy (<5%),energy (equation (3.17)). With a standard PMT on the backend of resolution (∼300 keV) and, for the first time, time resolution > (∼30–40 ps). The system is called MRSt that is based on the this diagnostic, the time resolution was limited to 100 ps, existing MRS principle and PDDT technology [236, 237]. which prompted an upgrade of the system by incorporating fi Current point-design includes a 40 μmthick,400μmdiameter the PDDT technology. The rst phase of the project involved CD foil positioned on the outside of a hohlraum for production the installation of the GCD into an insertion-well at a position [ ] of recoil deuterons from incident neutrons [238]; two dipoles of 3.9 m from the implosion 155 . The second phase and three quadrupoles, positioned outside the NIF target involved the incorporation of the PDDT technology to chamber for energy analysis and focusing of forward-scattered improve the time resolution to ∼10 ps [241, 242], and tests recoil deuterons onto a short focal plane of the spectrometer; of the GCD-PDDT performance [242]. and a PDDT with a CsI photocathode positioned at the focal plane. In the CsI photocathode, the recoil deuterons generate secondary electrons, which are subsequently accelerated by a 9.2.3. Gamma-ray spectroscopy. High-resolution gamma- spatially- and time-varying electric field that unskews and ray spectroscopy is a nuclear technique that has not been well- stretches the signal over a distance of ∼1m.Thissignalis utilized for ICF-diagnostic applications. The reason for this is subsequently amplified by an MCPs and detected by an array that the gamma-ray yields produced by the relevant nuclear of anodes. By dilating the pulse about 10–20 times, fast reactions are extremely low, orders of magnitude lower than temporal features (of order 10 ps) can be recorded and analyzed the main reaction branches (see equations (3.1)–(3.7) and

36 Plasma Phys. Control. Fusion 62 (2020) 023001 Topical Review

Figure 49. (Top) Schematic of the experimental setup for radiographing an indirect-drive implosion. For this experiment, five and ten laser beams that entered each end of the hohlraum with incident angles of 42° and 58.8° were used, respectively. The colors on the hohlraum wall illustrate the laser-intensity distribution. In this particular experiment, the charged-particle backlighter was driven by thirty laser beams, which delivered a laser energy of ∼11 kJ in a 1 ns square pulse. The backlighter produced d3He-protons that were used to backlight the holhraum. (Bottom) d3He-proton-fluence radiographs recorded at different times of the indirect-drive implosion (each image represents one implosion). The dark areas in the radiographs correspond to regions of higher proton fluence. From these radiographs, the path-integrated field distributions can be determined. From [206]. Adapted with permission from AAAS.

Figure 50. (Top) Schematic of the experimental setup for radiographing a direct-drive implosion. In this experiment, a series of 850 μm 3 diameter CH capsules (with a 35 μm wall thickness), filled with 15 am of H2 gas, were backlit by d He-protons. These CH capsules were driven with a subset of laser beams, delivered in a shaped pulse (OMEGA type RD1501p, which is shown at the bottom of the figure). From left to right, the images are from OMEGA shots 51244, 51246, 51247, and 51250. Adapted from [209], with the permission of AIP Publishing.

37 Plasma Phys. Control. Fusion 62 (2020) 023001 Topical Review (3.10)–(3.11)), which put stringent requirements on the more capable than those used during the preceding decade. efficiency and resolution. This is generally a consequence of the fact that the efforts, In addition to the dt-fusion gamma-rays and 4.44 MeV lessons learned, and experience gained on previous facilities gamma-rays from the 12C(n, nγ) reaction, there is a large range of enabled more sophisticated diagnostics on the next-generation gamma-raysthatcanbeusedtodiagnoseanICFimplosion.To ICF facilities. Higher fusion yields have also facilitated higher- that end, a high-efficiency, high-resolution gamma-ray spectro- quality measurements, which enabled an increased level of meter would open up a window for exploring new physics in an understanding about the physics governing the nuclear phase of ICF experiment. With adequate efficiency and energy resolution, an ICF implosion. This in turn provided essential guidance of the 15.58 MeV gamma-rays from the 2H(n, γ)3H reaction can be the ICF programs, resulting in further improved fusion yields. used to diagnose fuel ρR, and the 14.18 MeV and 17.87 MeV With the implementation of new high-fusion-yield facilities gamma-rays from 12C(n, γ)13C can also be used to diagnose the world-wide, the next-generation nuclear diagnostics will play ablator. In addition, short-range knock-on fuel ions and fusion an even more important role for decades to come. alphas that generate gamma-ray lines can be used to probe fuel- ablator mix. In particular, 4.91 MeV and 5.69 MeV gamma-ray lines from 13C(2H, n)14N; the 2.80 MeV, 3.40 MeV, 4.49 MeV, and 6.03 MeV gamma-ray lines from 9Be(2H, n)10B; and the Acknowledgments 4.40 MeV gamma-ray line from 9Be(4He, n)12Ccanbeusedfor this purpose. I would like to thank everyone who provided ideas and Although the 4-channel GRH has some energy-resolving research to this review, and to those who generously shared fi capabilities and that there are ideas and concepts for gamma- their gures. Any oversights or errors are my own. The work ( ray-spectroscopy for ICF applications [243], this is an area of described herein was supported in part by US DOE Grant ) ( research that needs to be advanced significantly. No. DE-FG03-03SF22691 , LLE subcontract Grant No. 412160-001G) and the Center for Advanced Nuclear Diag- nostics (Grant No. DE-NA0003868). 9.3. Charged-particle diagnostics As discussed in section 7, charged-particle diagnostics have been used extensively to diagnose ICF implosions with ρR- values up to about 200 mg cm−2. Although this area is at the ORCID iDs fore-front of diagnostic-development for ICF applications, // / there are new and exciting ideas that would open a new J A Frenje https: orcid.org 0000-0001-6846-0378 window to ICF implosions.

9.3.1. Radiography with multiple mono-energetic charged References particles. Mono-energetic charged-particle radiography, basedontheD3He gas-filled backlighter that generates 14.7 [ ] and 3.0 MeV protons, has been used with great success at 1 Nuckolls J H 2007 Contributions to the genesis and progress of ICF Inertial Confinement Nuclear Fusion: A Historical various facilities, as discussed in section 7.7.However,such Approach by its Pioneers (Guillermo Velarde and Natividad backlighter has some limitations particularly when it is being Santamaria, Foxwell & Davies (UK) Ltd ©) applied to an environment with strong fields and large ρR values [2] Nuckolls J H 1998 Early steps toward Inertial Confinement because of large deflections and scattering of 3.0 MeV protons. Fusion energy (IFE)(1952–1962) UCRL-ID-131075 Consequently, a new backlighter platform that will overcome Lawrence Livermore National Laboratory [3] Yonas G 2007 A pulsed power Inertial Confinement Fusion these challenges is currently being developed at OMEGA and journey Inertial Confinement Nuclear Fusion: A Historical the NIF [244]. This platform is based on a tri-particle, mono- Approach by its Pioneers (Guillermo Velarde and Natividad energetic backlighter that provides a 9.5 MeV deuterons in Santamaria, Foxwell & Davies (UK) Ltd ©) addition to the two 14.7 and 3.0 MeV protons from a DT3He [4] Bock R M 2007 Historical remarks on the heavy ion Inertial fi – gas-filled capsule implosion. The additional 9.5 MeV deuterons Con nement Fusion activities in Germany 1976 1985 Inertial Confinement Nuclear Fusion: A Historical will be important for backlighting experiments, allowing Approach by its Pioneers (Guillermo Velarde and Natividad discriminatory, high-quality radiographs of E and B fields and Santamaria, Foxwell & Davies (UK) Ltd ©) plasma matter. In general, three distinct mono-energetic particles [5] Nuckolls J, Wood L, Thiessen A and Zimmerman G 1972 are needed in radiography to distinguish between the three Laser compression of matter to super-high densities Nature ‘ ’ fi fi 239 139 unique effects of electric elds, magnetic elds, and plasma [ ] ( ) 6 Sasaki T et al 1978 Thermonuclear fusion study by glass laser matter essentially three equations for three unknowns . GEKKO II Tech. Rep. Osaka Univ. 28 169–83 [7] Azechi H et al 1978 Inertial Confinement Fusion research in Osaka Presented at 7th Conf. Plasma Phys. Controlled ( ( – 10. Concluding remarks Nuclear Fusion Research Innsbruck, Austria 23 30 August IAEA-CN-37/M-4 [8] Campbell P M, Charatis G and Montry G R 1975 Laser- Since the early 70s, each decade has displayed new ICF driven compresison of glass microspheres Phys. Rev. Lett. facilities and nuclear diagnostics that were orders of magnitude 34 74

38 Plasma Phys. Control. Fusion 62 (2020) 023001 Topical Review

[9] Slivinsky V W, Ahlstrom H G, Tirsell K G, Larsen J, Glaros S, [31] Zhao D, Wan L, Lin Z, Shao P and Zhu J 2015 SG-II-Up Zimmerman G and Shay H 1975 Measurement of the ion prototype final optics assembly: optical damage and clean- temperature in laser driven fusion Phys. Rev. Lett. 35 1083 gas control High Power Laser Sci. Eng. 3 e7 [10] Soures J M et al 1981 A review of high density, laser driven, [32] Zheng W et al 2016 High Power Laser Sci. Eng. 4 e21 implosion experiments at the laboratory for laser energetics [33] He X T, Zhang W Y and the Chinese ICF team 2013 Laser Interaction and Related Plasma Phenomena vol 5 ed Advances in the national inertial fusion program of China H J Schwarz et al (New York: Plenum) pp 463–81 EPJ Web Conf. 59 01009 [11] Mitchell K B, Van Hulsteyn D B, McCall G H, Lee P and [34] Garanin S G, Bel’kov S A and Bondarenko S V 2012 Greim H 1979 Compression measurements of neon filled Conception of UFL-2M laser facility 39th Zvenigorod Int. glass microballons irradiated by CO2 laser light Phys. Rev. Conf. on Plasma Physics and Controlled Fusion Lett. 42 232 (Zvenigorod, Russia, 6–10 February ) R-12 Perkins R B 1978 Recent progress in Inertical Confinement [35] Tabak M, Hammer J, Glinsky M E, Kruer W L, Wilks S C, Fusion Research at the Los Alamos scientific Laboratory Los Woodworth J, Campbell E M, Perry M D and Mason R J Alamos Scientific Laboratory Report LA-UR-78-1629 1994 Ignition and high gain with ultrapowerful lasers Phys. [12] Hunt J T and Speck D R 1989 Present and future performance Plasmas 1 1626 of the NOVA laser system Opt. Eng. 28 461 [36] Kodama R et al 2001 Fast heating of ultrahigh-density plasma [13] Yamanaka C 1985 Inertial Confinement Fusion research by as a step towards laser fusion ignition Nature 412 798 Gekko XII glass laser Fusion Technol. 8 179 [37] Shiraga H et al 2011 Fast ignition integrated experiments with [14] Thiell G, Adolf A, Andre M, Fleurot N, Friart D, Gekko and LFEX lasers Plasma Phys. Control. Fusion 53 Juraszek D and Schirmann D 1988 The PHEBUS 124029 experimental facility operating at 250 ps and 0.53 μm Laser [38] Azechi H et al 2013 Present status of fast ignition realization Part. Beams 6 93 experiment and inertial fusion energy development Nucl. [15] Kochemasov G G 1993 Laser fusion investigations on high- Fusion 53 104021 power photodissociation iodine lasers ISKRA Proc. SPIE [39] Batani D et al 2011 The HiPER project for Inertial 1980, Iodine Lasers and Applications Confinement Fusion and some experimental results on [16] Kirillov G A, Murugov V M, Punin V T and Shemyakin V I advanced ignition schemes Plasma Phys. Control. Fusion 53 1990 High power laser system ISKRA V Laser Part. Beams 124041 8 827 [40] Slutz S A, Herrmann M C, Vesey R A, Sefkow A B, [17] Zhao Y 1997 Development of ICF laser driver technology in Sinars D B, Rovang D C, Peterson K J and Cuneo M E 2010 China Proc. SPIE 3047, Solid State Lasers for Application Pulsed-power-driven cylindrical liner implosions of laser to Inertial Confinement Fusion: Second Annual Int. Conf. preheated fuel magnetized with an axial field Phys. Plasmas [18] Soures J M, Hutchison R J, Jacobs S D, Lund L D, 17 056303 McCrory R L and Richardson M C 1983 OMEGA: a short- [41] Gomez M R et al 2105 Demonstration of thermonuclear wavelength laser for fusion experiments Proc. 10th IEEE/ conditions in magnetized liner inertial fusion experiments NPSS Symp. on Fusion Engineering, SOFE’83 (New York: Phys. Plasmas 22 056306 IEEE) pp 1392–7 [42] Lindl J D, Amendt P, Berger R L, Glendinning S G, [19] Takabe H et al 1988 Scalings of implosion experiments for Glenzer S H, Haan S W, Kauffman R L, Landen O L and high neutron yield Phys. Fluids 31 2884 Suter L J 2004 The physics basis for ignition using indirect- [20] Campbell E M, Hunt J T, Bliss E S, Speck D R and Drake R P drive targets on the national ignition facility Phys. Plasmas 1986 Nova experimental facility Rev. Sci. Instrum. 57 2101 11 339 [21] McCrory R L et al 1988 Laser-driven implosion of [43] Edwards M J et al 2013 Phys. Plasmas 20 070501 thermonuclear fuel to 20 to 40 g cm−3 Nature 335 225 Moses E I and Boyd R N 2013 Progress towards ignition on [22] Boehly T R et al 1997 Initial performance results of the the national ignition facility Phys. Plasmas 20 070501 OMEGA laser system Opt. Commun. 133 495 [44] Freidberg J P 2007 Fusion Energy and Plasma Physics [23] 1997 Review of the department of energy’s Inertial (Cambridge: Cambridge University Press) Confinement Fusion program, the national ignition facility [45] Betti R, Anderson K S, Chang P Y, Nora R, Zhou C D, Committee for the Review of the Department of Energy’s Spears B, Edwards J, Haan S W and Lindl J 2010 Inertial Confinement Fusion Program, Commission on Thermonuclear ignition in Inertial Confinement Fusion and Physical Sciences, Mathematics, and Applications, National comparison with magnetic confinement Phys. Plasmas 17 Research Council (Washington, DC: National Academic 058102 Press)(http://nap.edu/catalog/5730.html) [46] Bose A, Betti R, Shvarts D and Woo K M 2017 The physics [24] Miller G H, Moses E I and Wuest C R 2004 The National of long- and intermediate-wavelength asymmetries of the hot Ignition Facility: enabling fusion ignition for the 21st spot: compression hydrodynamics and energetics Phys. century Nucl. Fusion 44 228 Plasmas 24 102704 [25] André M L 1999 The french megajoule laser project (LMJ) [47] Bodner S E 1974 Rayleigh–Taylor instability and laser-pellet Fusion Eng. Des. 44 43 fusion Phys. Rev. Lett. 33 761 [26] Lindl J, Landen O, Edwards J, Moses E D and NIC Team [48] Smalyuk V A, Hu S X, Hager J D, Delettrez J A, 2014 Review of the national ignition campaign 2009–2012 Meyerhofer D D, Sangster T C and Shvarts D 2009 Phys. Plasmas 21 020501 Spherical Rayleigh–Taylor growth of three-dimensional [27] Kritcher A L et al 2016 Integrated modeling of cryogenic layered broadband perturbations on OMEGA Phys. Plasmas 16 highfoot experiments at the NIF Phys. Plasmas 23 052709 112701 [28] Le Pape S et al 2018 Fusion energy output greater than the [49] Smalyuk V A, Xu S X, Hager J D, Delettrez J A, kinetic energy of an imploding shell at the national ignition Meyerhofer D D, Sangster T C and Shvarts D 2009 facility Phys. Rev. Lett. 120 245003 Rayleigh-Taylor growth measurements in the acceleration [29] Sangster T C et al 2010 Shock-tuned cryogenic DT-implosion phase of spherical implosions on OMEGA Phys. Rev. Lett. performance on OMEGA Phys. Plasmas 17 056312 103 105001 [30] Regan S P et al 2016 Demonstration of fuel hot-spot pressure [50] Frenje J A et al 2013 Diagnosing implosion performance at in excess of 50 Gbar for direct-drive, layered deuterium- the National Ignition Facility (NIF) by means of neutron tritium implosions on OMEGA Phys. Rev. Lett. 117 025001 spectrometry Nucl. Fusion 53 043014

39 Plasma Phys. Control. Fusion 62 (2020) 023001 Topical Review

[51] Fisher R K, Parks P B, McChesney J M and Rosenbluth M N [72] Frenje J A, Grabowski P E, Li C K, Séguin F H, Zylstra A B, 1994 Fast alpha particle diagnostics using knock-on ion tails Gatu Johnson M, Petrasso R D, Yu Glebov V and Nucl. Fusion 34 1291 Sangster T C 2015 Measurements of ion stopping around the [52] Ballabio L, Gorini G and Källne J 1997 α-particle knock-on bragg peak in high energy-density plasmas Phys. Rev. Lett. signature in the neutron emission of DT plasmas Phys. Rev. 115 205001 E 55 3358 [73] Frenje J A et al 2019 Experimental validation of low-Z ion- [53] Källne J, Ballabio L, Frenje J, Conroy S, Ericsson G, stopping formalisms around the bragg peak in high-energy- Tardocchi M, Traneus E and Gorini G 2000 Observation of density plasmas Phys. Rev Lett. 122 015002 the alpha particle ‘Knock-On’ neutron emission from [74] Azechi H et al 1986 Experimental determination of fuel magnetically confined DT fusion plasmas Phys. Rev. Lett. density‐radius product of Inertial Confinement Fusion 85 1246 targets using secondary nuclear fusion reactions Appl. Phys. [54] Petrasso R D et al 1996 Measuring implosion symmetry and Lett. 49 555 core conditions in the national ignition facility Phys. Rev. [75] Azechi H, Stapf R O, Miyanaga N, Tsuji R, Yamanaka M, Lett. 77 2718 Ido S, Nishihara K, Yabe T and Yamanaka C 1987 Study of [55] Casey D T et al 2017 Thermonuclear reactions probed at fuel-pusher mixing in laser-driven implosions, using stellar-core conditions with laser-based inertial-confinement secondary nuclear fusion reactions Phys. Rev. Lett. 59 2635 fusion Nat. Phys. 13 1227–31 [76] Azechi H, Cable M D and Stapf R O 1991 Review of [56] Brysk H 1973 Fusion neutron energies and spectra Plasma secondary and tertiary reactions, and neutron scattering as Phys. 15 611 diagnostic techniques for Inertial Confinement Fusion [57] Ballabio L, Kallne J and Gorini G 1998 Relativistic targets Laser Part. Beams 9 119 calculation of fusion product spectra for thermonuclear [77] Cable M D and Hatchett S P 1987 Neutron spectra from plasmas Nucl. Fusion 38 1723 Inertial Confinement Fusion targets for measurement of fuel [58] Appelbe B and Chittenden J 2011 The production spectrum areal density and charged particle stopping powers J. Appl. in fusion plasmas Plasma Phys. Control. Fusion 53 Phys. 62 2233 045002 [78] Séguin F H et al 2002 Using secondary-proton spectra to [59] Munro D H 2016 Interpreting inertial fusion neutron spectra study the compression and symmetry of deuterium-filled Nucl. Fusion 56 036001 capsules at OMEGA Phys. Plasmas 9 2725 [60] Gatu Johnson M et al 2013 Measurements of collective fuel [79] Kurebayashi S et al 2005 Using nuclear data and Monte Carlo velocities in deuterium-tritium exploding pusher and techniques to study areal density and mix in D2 implosions cryogenically layered deuterium-tritium implosions on the Phys. Plasmas 12 032703 NIF Phys. Plasmas 20 042707 [80] Rinderknecht H G et al 2015 Using multiple secondary fusion [61] Munro D H 2017 Impact of temperature-velocity distribution products to evaluate fuel ρR, electron temperature, and mix on fusion neutron peak shape Phys. Plasmas 24 056301 in deuterium-filled implosions at the NIF Phys. Plasmas 22 [62] Murphy T J 2014 The effect of turbulent kinetic energy on 082709 inferred ion temperature from neutron spectra Phys. Plasmas [81] Springer P T et al 2019 A 3D dynamic model to assess 21 072701 impacts of low-mode asymmetry, aneurysms, and mix [63] Frenje J A et al 2011 Measurements of the differential cross induced radiative loss on capsule performance across ICF section for the elastic n-3H and n-2H scattering at 14.1 MeV platforms Nucl. Fusion 59 032009 by using an Inertial Confinement Fusion (ICF) facility Phys. [82] Rosenberg M J et al 2014 Exploration of the transition from Rev. Lett. 107 122502 the hydrodynamiclike to the strongly kinetic regime in [64] Allred J C, Armstrong A H and Rosen L 1953 The interaction shock-driven implosions Phys. Rev. Lett. 112 185001 of 14 MeV neutrons with protons and deuterons Phys. Rev. [83] Rinderknecht H G et al 2014 Kinetic mix mechanisms in 91 90 shock-driven Inertial Confinement Fusion implosions Phys. [65] Dave J H and Gould C R 1983 Optical model analysis of Plasmas 21 056311 scattering of 7 to 15 MeV neutrons from 1−p shell nuclei [84] Rinderknecht H G et al 2015 Ion thermal decoupling and Phys. Rev. C 28 2212 species separation in shock-driven implosions Phys. Rev. [66] Knoll G F 2010 Radiation Detection and Measurement 4th Lett. 114 025001 edn (New York: Wiley) [85] Hoffman N M et al 2015 Approximate models for the ion- [67] Frenje J A, Li C K, Séguin F H, Casey D T, Petrasso R D, kinetic regime in inertial-confinement-fusion capsule Sangster T C, Betti R, Glebov V Y and Meyerhofer D D implosions Phys. Plasmas 22 052707 2009 Diagnosing fuel ρR and ρR asymmetries in cryogenic [86] Rosenberg M J et al 2015 Assessment of ion kinetic effects in deuterium-tritium implosions using charged-particle shock-driven Inertial Confinement Fusion implosions using spectrometry at OMEGA Phys. Plasmas 16 042704 fusion burn imaging Phys. Plasmas 22 062702 [68] Atzeni S and Meyer-Ter-Vehn J 2004 The physics of inertial [87] Sio H et al 2019 Observations of multiple nuclear reaction fusion: beam plasma interaction, hydrodynamics, hot dense histories and fuel-ion species dynamics in Shock-Driven matter International Series of Monographs on Physics Inertial Confinement Fusion implosions Phys. Rev. Lett. 122 (Oxford: Oxford University Press) 035001 [69] Brown L S et al 2005 Charged particle motion in a highly [88] Li C K et al 2006 Monoenergetic proton backlighter for ionized plasma Phys. Rep. 410 237 measuring E and B fields and for radiographing implosions [70] Li C K and Petrasso R D 1993 Charged-particle stopping and high-energy density plasmas Rev. Sci. Instrum. 77 powers in Inertial Confinement Fusion plasmas Phys. Rev. 10E725 Lett. 70 3059 [89] Li C K et al 2006 Measuring E and B fields in laser-produced Li C K and Petrasso R D 2015 Erratum: charged-particle plasmas with monoenergetic proton radiography Phys. Rev. stopping powers in Inertial Confinement Fusion plasmas Lett. 97 135003 [Phys. Rev. Lett. 70, 3059 (1993)] Phys. Rev. Lett. 114 [90] Rygg J R et al 2008 Supplementary material: proton 199901 radiography of inertial fusion implosions Science 319 1223 [71] Maynard G et al 1985 Born random phase approximation for [91] Li C K et al 2008 Monoenergetic-proton-radiography ion stopping in an arbitrarily degenerate electron fluid measurements of implosion dynamics in direct-drive J. Physique 46 1113 inertial-confinement fusion Phys. Rev Lett. 100 225001

40 Plasma Phys. Control. Fusion 62 (2020) 023001 Topical Review

[92] Li C K et al 2009 Study of direct-drive capsule implosions in [113] Nagai T et al 2014 The development of the neutron detector Inertial Confinement Fusion with proton radiography for the fast ignition experiment by using LFEX and Gekko Plasma Phys. Control. Fusion 51 014003 XII facility Plasma Fusion Res. 9 4404105 [93] Hahn K D, Ruiz C L, Cooper G W, Nelson A J, [114] Johnson M G et al 2012 Neutron spectrometry—an essential Chandler G A, Leeper R J, McWatters B R, tool for diagnosing implosions at the national ignition Smelser R M and Torres J A 2012 Calibration of neutron- facility Rev. Sci. Instrum. 83 10D308 yield diagnostics in attenuating and scattering environments [115] Sayre D B et al 2013 Measurement of the T+T neutron Rev. Sci. Instrum. 83 10D914 spectrum using the national ignition facility Phys. Rev. Lett. [94] Ruiz C L, Leeper R J, Schmidlapp F A, Cooper G and 111 052501 Malbrough D J 1992 Absolute calibration of a total yield [116] Johnson M G et al 2018 Experimental evidence of a variant indium activation detector for DD and DT neutrons Rev. Sci. neutron spectrum from the T(t,2n)4He reaction at center-of- Instrum. 63 4889 mass energies in the range of 16–50 keV Phys. Rev. Lett. [95] Barnes C W, Murphy T J and Oertel J A 2001 High-yield 121 042501 neutron activation system for the national ignition facility [117] Hatarik R et al 2015 Analysis of the neutron time-of-flight Rev. Sci. Instrum. 72 818 spectra from Inertial Confinement Fusion experiments [96] Bleuel D L et al 2012 Neutron activation diagnostics at the J. Appl. Phys. 118 184502 national ignition facility Rev. Sci. Instrum. 83 10D313 [118] Cerjan C J et al 2018 Dynamic high energy density plasma [97] Yeamans C B, Bleuel D L and Bernstein L A 2012 Enhanced environments at the national ignition facility for nuclear NIF neutron activation diagnostics Rev. Sci. Instrum. 83 science research J. Phys. G: Nucl. Part. Phys. 45 033003 10D315 [119] Hatarik R, Bernstein L A, Caggiano J A, Carman M L, [98] Lerche R A, McLerran W R and Tripp G R 1976 UCRL Schneider D H G, Zaitseva N P and Wiedeking M 2012 Report No. 50021-76 Lawrence Livermore National Characterizing time decay of bibenzyl scintillator using time Laboratory, Livermore, CA correlated single photon counting Rev. Sci. Instrum. 83 [99] Cooper G W et al 2012 Copper activation deuterium-tritium 10D911 neutron yield measurements at the national ignition facility [120] Yu Glebov V et al 2012 Testing a new NIF neutron time-of- Rev. Sci. Instrum. 83 10D918 flight detector with a bibenzyl scintillator on OMEGA Rev. [100] Hahn K D, Cooper G W, Ruiz C L, Fehl D L, Chandler G A, Sci. Instrum. 83 10D309 Knapp P F, Leeper R J, Nelson A J, Smelser R M and [121] Kabadi N V et al 2016 Sensitivity of chemical vapor Torres J A 2014 Fusion-neutron-yield, activation deposition diamonds to DD and DT neutrons at OMEGA measurements at the Z accelerator: design, analysis, and and the national ignition facility Rev. Sci. Instrum. 87 sensitivity Rev. Sci. Instrum. 85 043507 11D817 [101] Cooper G W and Ruiz C L 2001 NIF total neutron yield [122] Rinderknecht H G et al 2012 A novel particle time of flight diagnostic Rev. Sci. Instrum. 72 814 diagnostic for measurements of shock- and compression- [102] Landoas O et al 2011 Absolute calibration method for laser bang times in D3He and DT implosions at the NIF Rev. Sci. megajoule neutron yield measurement by activation Instrum. 83 10D902 diagnostics Rev. Sci. Instrum. 82 073501 [123] Tietbohl G L, Cable M D, Nelson M B, Bennett C E, Mant G, [103] Caillaud T et al 2016 Recent advance in target diagnostics on Deane D J and Connolly N A 1990 LANSA: a large neutron the Laser MégaJoule (LMJ) Proc. SPIE 9966, Target scintillator array for neutron spectroscopy at Nova Rev. Sci. Diagnostics Physics and Engineering for Inertial Instrum. 61 3231 Confinement Fusion V 996606 [124] Chrien R E, Simmons D F and Holmberg D L 1992 Neutron [104] Yeamans C B and Bleuel D L 2016 The spatially distributed time‐of‐flight ion temperature diagnostic for inertial‐ neutron activation diagnostic FNADs at the national ignition confinement‐fusion experiments Rev. Sci. Instrum. 63 4886 facility Fusion Sci. Technol. 72 120 [125] Knauer J P, Kremens R L, Russotto M A and Tudman S 1995 [105] Casey D T et al 2015 Improved performance of high areal Using cosmic rays to monitor large scintillator arrays Rev. density indirect drive implosions at the national ignition Sci. Instrum. 66 926 facility using a Four-Shock adiabat shaped drive Phys. Rev. [126] Izumi N, Yamagajo T, Nakano T, Kasai T, Azechi H, Lett. 115 105001 Kato Y and Nakai S 1996 Development of multi channel [106] Zifeng S, Jiabin C, Zhongjie L, Xiayu Z and Qi T 2017 The neutron spectrometer at GEKKO XII laser fusion facility calibration of DD neutron indium activation diagnostic AIP Conf. Proc. 369 550 Plasma Sci. Technol. 17 337 [127] Gamalii E G, Yu Gus’Kov S, Krokhin O N and Ruzanov V B [107] Zifeng S, Jiabin C, Zhongjie L, Xiayu Z and Qi T 2014 1975 Possibility of determining the characteristics of laser Evaluation of uncertainty in DD neutron yield diagnosis by plasma by measuring the neutrons of the DT reactions JETP indium activation High Power Laser Part. Beams 26 1165 Lett. 21 70 [108] Glebov V Y, Stoeckl C, Sangster T C, Meyerhofer D D, [128] Cable M D and Hatchett S P 1987 Neutron spectra from Radha P B, Padalino S, Baumgart L, Colburn R and Inertial Confinement Fusion targets for measurement of fuel Fuschino J 2003 Carbon activation diagnostic for tertiary areal density and charged particle stopping powers J. Appl. neutron measurements Rev. Sci. Instrum. 74 1717 Phys. 62 2233 [109] Kilkenny J D 1992 Recent diagnostic developments at Nova [129] Lerche R A, Hatchett S P, Cable M D, Nelson M B and Rev. Sci. Instrum. 63 4688 Murphy T J 1992 Plasma temperatures from first-hit neutron [110] Murphy T J, Jimerson J L, Berggren R R, Faulkner J R, time-of-flight spectra Rev. Sci. Instrum. 63 4877 Oertel J A and Walsh P J 2001 Neutron time-of-flight and [130] Chrien R E, Klare K A and Murphy T J 1997 Measurements emission time diagnostics for the national ignition facility of neutron spectra from Nova targets Rev. Sci. Instrum. Rev. Sci. Instrum. 72 850 68 607 [111] Yu Glebov V et al 2010 The national ignition facility neutron [131] Lerche R A, Phillion D W and Tietbohl G L 1995 25 ps time-of-flight system and its initial performance Rev. Sci. neutron detector for measuring ICF‐target burn history Rev. Instrum. 81 10D325 Sci. Instrum. 66 933 [112] Chen Z et al 2018 Ion temperature measurements of indirect- [132] Stoeckl C, Yu Glebov V, Roberts S, Sangster T C, drive implosions with the neutron time-of-flight detector on Lerche R A, Griffith R L and Sorce C 2003 Ten-inch SG-III laser facility Rev. Sci, Instrum. 89 023504 manipulator-based neutron temporal diagnostic for

41 Plasma Phys. Control. Fusion 62 (2020) 023001 Topical Review

cryogenic experiments on OMEGA Rev. Sci. Instrum. with the magnetic recoil spectrometer at OMEGA and the 74 1713 NIF Rev. Sci. Instrum. 82 073502 [133] Frenje J A et al 2004 Measuring shock-bang timing and ρR [152] Zhang J et al 2015 Development of a compact magnetic evolution of D3He implosions at OMEGA Phys. Plasmas proton recoil spectrometer for measurement of deuterium- 11 2798 tritium neutrons Rev. Sci. Instrum. 86 125115 [134] Sio H et al 2016 A particle x-ray temporal diagnostic (PXTD) [153] Kim Y et al 2012 D-T gamma-to-neutron branching ratio for studies of kinetic, multi-ion effects, and ion-electron determined from Inertial Confinement Fusion plasmas Phys. equilibration rates in Inertial Confinement Fusion plasmas at Plasmas 19 056313 OMEGA Rev. Sci. Instrum. 87 11D701 [154] Herrmann H W et al 2010 Diagnosing Inertial Confinement [135] Guler N, Volegov P, Danly C R, Grim G P, Merrill F E and Fusion gamma ray physics Rev. Sci. Instrum. 81 10D333 Wilde C H 2012 Simultaneous usage of pinhole and [155] Herrmann H W et al 2016 Next generation gamma-ray penumbral apertures for imaging small scale neutron sources Cherenkov detectors for the national ignition facility Rev. from Inertial Confinement Fusion experiments Rev. Sci. Sci. Instrum. 87 11E732 Instrum. 83 10D316 [156] Rubery M S et al 2010 GEANT4 simulations of Cherenkov [136] Ress D, Lerche R A, Ellis R I and Lane S M 1988 Rev. Sci. reaction history diagnostics Rev. Sci. Instrum. 81 10D328 Instrum. 59 1694 [157] Hoffman N M, Wilson D C, Herrmann H W and Young C S Ress D, Lerche R A, Ellis R J, Heaton G W and Lehr D E 1995 2010 Using gamma-ray emission to measure areal density of High-sensitivity scintillating-fiber imaging detector for high- Inertial Confinement Fusion capsules Rev. Sci. Instrum. 81 energy neutrons Rev. Sci. Instrum. 66 4943 10D332 [137] Disdier L, Lerche R A, Bourgade J L and Yu Glebov V 2004 [158] Cerjan C, Sayre D B, Landen O L, Church J A, Stoeffl W, Capillary detector with deuterated scintillator for Inertial Grafil E M, Herrmann H W, Hoffman N M and Kim Y 2015 Confinement Fusion neutron images Rev. Sci. Instrum. Gamma reaction history ablator areal density constraints 75 2134 upon correlated diagnostic modeling of national ignition [138] Caillaud T et al 2012 Development of the large neutron facility implosion experiments Phys. Plasmas 22 032710 imaging system for Inertial Confinement Fusion experiments [159] Herrmann H W et al 2018 Progress on next generation Rev. Sci. Instrum. 83 033502 gamma-ray Cherenkov detectors for the national ignition [139] Ampleford D J et al 2018 One dimensional imager of facility Rev. Sci. Instrum. 89 10I148 neutrons on the Z machine Rev. Sci. Instrum. 89 10I132 [160] Zylstra A B, Herrmann H W, Kim Y H, McEvoy A M, [140] Grim G P et al 2013 Nuclear imaging of the fuel assembly in Schmitt M J, Hale G, Forrest C, Yu Glebov V and Stoeckl C ignition experiments Phys. Plasmas 20 056320 2017 Simultaneous measurement of the HT and DT fusion [141] Merrill F E et al 2012 The neutron imaging diagnostic at NIF burn histories in inertial fusion Implosions Rev. Sci. Instrum. (invited) Rev. Sci. Instrum. 83 10D317 88 053504 [142] Fatherley V E, Batha S H, Danly C R, Goodwin L A and [161] Zylstra A B, Herrmann H W, Kim Y H, Meaney K, Herrmann H W 2017 System design of the NIF neutron Geppert-Kleinrath H, Schmitt M J, Hoffman N M, imaging system north pole Proc. SPIE 10390, Target Leatherland A and Gales S 2018 Cherenkov detector Diagnostics Physics and Engineering for Inertial analysis for implosions with multiple nuclear reactions Rev. Confinement Fusion VI 103900F Sci. Instrum. 89 10I103 [143] Frenje J A et al 2002 Absolute measurements of neutron [162] Herrmann H W et al 2014 Extended performance gas yields in DD and DT implosions at the OMEGA laser Cherenkov detector for gamma-ray detection in high-energy facility using CR-39 track detectors Rev. Sci. Instrum. density experiments Rev. Sci. Instrum. 85 11E124 73 2597 [163] Kim Y, Herrmann H W, McEvoy A M, Young C S, [144] Lahmann B, Milanese L M, Han W, Gatu Johnson M, Hamilton C, Schwellenbach D D, Malone R M, Séguin F H, Frenje J A, Petrasso R D, Hahn K D and Kaufman M I and Smith A S 2016 Aerogel Cherenkov Jones B 2016 Application of the coincidence counting detector for characterizing the intense flash x-ray source, technique to DD neutron spectrometry data at the NIF, Cygnus, spectrum Rev. Sci. Instrum. 87 11E723 OMEGA, and Z Rev. Sci. Instrum. 87 11D801 [164] Séguin F H et al 2003 Spectrometry of charged particles from [145] Frenje J A et al 2001 A neutron spectrometer for precise inertial-confinement-fusion plasmas Rev. Sci. Instrum. 74 975 measurements of DT neutrons from 10 to 18 MeV at [165] Séguin F H et al 2002 Measurements of ρR asymmetries at OMEGA and the national ignition facility Rev. Sci. Instrum. burn time in inertial-confinement-fusion capsules Phys. 72 854 Plasmas 9 3558 [146] Frenje J A et al 2008 First measurements of the absolute [166] Li C K et al 2002 Effects of fuel-shell mix upon direct-drive, neutron spectrum using the magnetic recoil spectrometer at spherical implosions on OMEGA Phys. Rev. Lett. 89 165002 OMEGA Rev. Sci. Instrum. 79 10E502 [167] Petrasso R D et al 2003 Measuring implosion dynamics [147] Frenje J A et al 2010 Probing high areal-density (ρR) through ρR evolution in inertial-confinement fusion cryogenic DT implosions using down scattered neutron experiments Phys. Rev. Lett. 90 095002 spectra measured by the magnetic recoil spectrometer [168] Smalyuk V A et al 2003 Time-resolved areal density (MRS) Phys. Plasmas 17 056311 measurements with proton spectroscopy in spherical [148] Casey D T et al 2012 Measuring the absolute deuterium– implosions Phys. Rev. Lett. 90 135002 tritium neutron yield using the magnetic recoil spectrometer [169] Li C K et al 2003 Capsule areal density asymmetries inferred at OMEGA and the NIF Rev. Sci. Instrum. 83 10D912 from 14.7 MeV protons in direct-drive OMEGA implosions [149] Gatu Johnson M et al 2014 Measurements of fuel and ablator Phys. Plasmas 10 1919 ρR in symmetry-capsule implosions with the magnetic recoil [170] Li C K et al 2004 Effects of fuel-shell mix upon direct-drive, neutron spectrometer (MRS) on the national ignition facility spherical implosions on OMEGA Phys. Rev. Lett. 92 Rev. Sci. Instrum. 85 11E104 205001 [150] X-5 Monte Carlo Team 2003 MCNP-A general Monte Carlo [171] Rygg J R et al 2006 Tests of the hydrodynamic equivalence 3 n-particle transport code Los Alamos National Laboratory of direct-drive implosions with different D2 and He Report No. LA-UR-03-1987 Version 5 mixtures Phys. Plasmas 13 052702 [151] Casey D T et al 2011 The coincidence counting technique for [172] Wilson D C, Singleton R L Jr, Grondalski J P, Hoffman N M, orders of magnitude background reduction in data obtained Nobile A Jr, Séguin F H, Frenje J A, Li C K and

42 Plasma Phys. Control. Fusion 62 (2020) 023001 Topical Review

Petrasso R D 2006 Diagnosing ablator burn through in [195] Sinenian N et al 2012 Heavy-ion emission from short-pulse 3 ignition capsules using D2+ He gas filled surrogates Rev. laser-plasma interactions with thin foils Phys. Plasmas 19 Sci. Instrum. 77 10E711 093118 [173] Zhou C D et al 2006 High-ρR implosions for fast-ignition fuel [196] Séguin F H et al 2004 D3He-proton emission imaging for assembly Phys. Rev. Lett. 98 025004 inertial-confinement-fusion experiments Rev. Sci. Instrum. [174] Rygg J R et al 2007 Nuclear measurements of fuel-shell mix 75 3520 in Inertial Confinement Fusion implosions at OMEGA Phys. [197] DeCiantis J L et al 2006 Proton core imaging of the nuclear Plasmas 14 056306 burn in Inertial Confinement Fusion implosions Rev. Sci. [175] Delamater N D et al 2008 Use of d-3He proton spectroscopy Instrum. 77 043502 as a diagnostic of shell ρR in capsule implosion experiments [198] Séguin F H et al 2006 Measured dependence of nuclear burn with ∼0.2 NIF scale high temperature Hohlraums at Omega region size on implosions parameters in Inertial Rev. Sci. Instrum. 79 10E526 Confinement Fusion experiments Phys. Plasmas 13 082704 [176] Zylstra A B et al 2012 Charged-particle spectroscopy for [199] Séguin F H et al 2016 Effects of fuel-capsule shimming and diagnosing shock ρR and strength in NIF implosions Rev. drive asymmetry on inertial-confinement-fusion symmetry Sci. Instrum. 83 10D901 and yield Phys. Plasmas 23 032705 [177] Séguin F H et al 2002 Diagnostic use of secondary d-3He [200] Li C K et al 2007 Observation of the decay dynamics and proton spectra for dd OMEGA targets Phys. Plasmas 9 2275 instabilities of megagauss field structures in laser-produced [178] Sangster T C et al 2003 Direct-drive cryogenic target plasmas Phys. Rev Lett. 99 015001 implosion performance on OMEGA Phys. Plasmas 10 1937 [201] Li C K, Séguin F H, Frenje J A, Rygg J R, Petrasso R D, [179] McKenty P W et al 2004 Direct-drive cryogenic target Town R P J, Landen O L, Knauer J P and Smalyuk V A implosion performance on OMEGA Phys. Plasmas 11 2790 2007 Observation of megagauss-field topology changes due [180] Sangster T C et al 2007 Cryogenic DT and D2 targets for to magnetic reconnection in laser-produced plasmas Phys. Inertial Confinement Fusion Phys. Plasmas 14 058101 Rev Lett. 99 055001 [181] Theobald W et al 2008 Initial experiments on the shock- [202] Rygg J R et al 2008 Proton radiography of inertial fusion ignition Inertial Confinement Fusion concept Phys. Plasmas implosions Science 319 1223 15 056306 [203] Li C K et al 2009 Proton radiography of dynamic electric and [182] Goncharov V N et al 2008 Performance of direct-drive magnetic fields in laser-produced high-energy-density cryogenic targets on OMEGA Phys. Plasmas 15 056310 plasmas Phys. Plasmas 16 056304 [183] Sangster T C et al 2008 High-areal-density fuel assembly in [204] Li C K et al 2009 Observations of electromagnetic fields and direct-drive cryogenic implosions Phys. Rev Lett. 100 plasma flow in hohlraums with proton radiography Phys. 185006 Rev Lett. 100 205001 [184] Stoeckl C et al 2008 Fast-ignition target design and [205] Li C K et al 2009 Pressure-driven, resistive experimental-concept validation on OMEGA Plasma Phys. magnetohydrodynamic interchange instabilities in laser- Control. Fusion 50 124044 produced high-energy-density plasmas Phys. Rev. E 80 [185] Theobald W et al 2009 Advanced-ignition-concept exploration 016407 on OMEGA Plasma Phys. Control. Fusion 51 124052 [206] Li C K et al 2010 Charged-particle probing of x-ray-driven [186] Li C K et al 2010 Diagnosing indirect-drive inertial- inertial-fusion implosions Science 327 1231 confinement-fusion implosions with charged particles [207] Li C K et al 2012 Impeding hohlraum plasma stagnation in Plasma Phys. Control. Fusion 52 124027 inertial-confinement Fusion Phys. Rev. Lett. 108 025001 [187] Frenje J A et al 2002 Studies of fuel and shell areal densities [208] Manuel M J-E, Sinenian N, Séguin F H, Li C K, Frenje J A, of OMEGA capsule implosions using scattered protons Rinderknecht H G, Casey D T, Zylstra A B, Phys. Plasmas 9 4719 Petrasso R D and Beg F N 2012 Mapping return currents in [188] Frenje J A et al 2009 Diagnosing ablator ρR and ρR laser-generated Z-pinch plasmas using proton Appl. Phys. asymmetries in capsule implosions using charged-particle Lett. 100 203505 spectrometry at the national ignition facility Phys. Plasmas [209] Séguin F H et al 2012 Time evolution of filamentation and 16 022702 self-generated fields in the coronae of directly driven [189] Frenje J A, Li C K, Séguin F H, Casey D T, Petrasso R D, inertial-confinement fusion capsules Phys. Plasmas 19 Sangster T C, Betti R, Yu Glebov V and Meyerhofer D D 012701 2009 Diagnosing fuel ρR and ρR asymmetries in cryogenic [210] Manuel M J-E et al 2012 First measurements of Rayleigh– deuterium-tritium implosions using charged-particle Taylor-induced magnetic fields in laser-produced plasmas spectrometry at OMEGA Phys. Plasmas 16 042704 Phys. Rev. Lett. 108 255006 [190] Zhang X et al 2017 Preliminary diagnosis of areal density in [211] Rosenberg M J, Ross J S, Li C K, Town R P J, Séguin F H, the deuterium fuel capsule by proton measurement at SG-III Frenje J A, Froula D H and Petrasso R D 2012 facility Proc. SPIE 10173 101730T Characterization of single and colliding laser-produced [191] Zhang X et al 2015 The application of proton spectrometers at plasma bubbles using Thomson scattering and proton the SG-III facility for ICF implosion areal density radiography Phys. Rev. E 86 056407 diagnostics High Power Laser Sci. Eng. 3 e28 [212] Manuel M J-E, Zylstra A B, Rinderknecht H G, Casey D T, [192] Rinderknecht H G et al 2014 A magnetic particle time-of- Rosenberg M J, Sinenian N, Li C K, Frenje J A, flight (MagPTOF) diagnostic for measurements of shock- Séguin F H and Petrasso R D 2012 Source characterization and compression-bang time at the NIF Rev. Sci. Instrum. 85 and modeling development for monoenergetic-proton 11D901 radiography experiments on OMEGA Rev. Sci. Instrum. 83 [193] Freeman C G et al 2011 Calibration of a Thomson parabola ion 063506 spectrometer and Fujifilm imaging plate detectors for protons, [213] Manuel M J-E et al 2012 Rayleigh–Taylor-induced magnetic deuterons, and alpha particles Rev. Sci. Instrum. 82 073301 fields in laser-irradiated plastic foils Phys. Plasmas 19 [194] Cobble J A, Flippo K A, Offermann D T, Lopez F E, 082710 Oertel J A, Mastrosimone D, Letzring S A and Sinenian N [214] Li C K et al 2013 Proton imaging of hohlraum plasma 2011 High-resolution Thomson parabola for ion analysis stagnation in inertial-confinement-fusion experiments Nucl. Rev. Sci. Instrum. 82 113504 Fusion 53 073022

43 Plasma Phys. Control. Fusion 62 (2020) 023001 Topical Review

[215] Li C K et al 2013 Structure and dynamics of colliding plasma [231] Glebov V Y et al 2018 Testing a Cherenkov neutron time-of- jets Phys. Rev. Lett. 111 235003 flight detector on OMEGA Rev. Sci. Instrum. 89 10I122 [216] Manuel M J-E et al 2013 Instability-driven electromagnetic [232] Batha S H, Volegov P L, Fatherley V E, fields in coronal plasmas Phys. Plasmas 20 056301 Geppert-Kleinrath V and Wilde C H 2018 Optimizing [217] Rosenberg M J et al 2014 Empirical assessment of the neutron imaging line of sight locations for maximizing detection efficiency of CR-39 at high proton fluence and a sampling of the cold fuel density in Inertial Confinement compact, proton detector for high-fluence applications Rev. Fusion implosions at the national ignition facility Rev. Sci. Sci. Instrum. 85 043302 Instrum. 89 10I147 [218] Manuel M J-E et al 2015 Collisional effects on Rayleigh– [233] Casey D T, Volegov P L, Merrill F E, Munro D H, Grim G P, Taylor-induced magnetic fields Phys. Plasmas 22 056305 Landen O L, Spears B K, Fittinghoff D N, Field J E and [219] Rosenberg M J, Li C K, Fox W, Igumenshchev I, Séguin F H, Smalyuk V A 2016 Fluence-compensated down-scattered Town R P J, Frenje J A, Stoeckl C, Glebov V and neutron imaging using the neutron imaging system at the Petrasso R D 2015 A laboratory study of asymmetric national ignition facility Rev. Sci. Instrum. 87 11E715 magnetic reconnection in strongly driven plasmas Nat. [234] Volegov P L, Danly C R, Merrill F E, Simpson R and Commun. 6 6190 Wilde C H 2015 On three-dimensional reconstruction of a [220] Rosenberg M J, Li C K, Fox W, Zylstra A B, Stoeckl C, neutron/x-ray source from very few two-dimensional Séguin F H, Frenje J A and Petrasso R D 2015 Slowing of projections J. Appl. Phys. 118 205903 magnetic reconnection concurrent with weakening plasma [235] Volegov P L, Danly C R, Fittinghoff D, Geppert-Kleinrath V, inflows and increasing collisionality in strongly driven laser- Grim G, Merrill F E and Wilde C H 2017 Three-dimensional plasma experiments Phys. Rev. Lett. 114 205004 reconstruction of neutron, gamma-ray, and x-ray sources [221] Li C K et al 2016 Scaled laboratory experiments explain the using spherical harmonic decomposition J. Appl. Phys. 122 kink behavior of the Crab Nebula jet Nat. Commun. 7 13081 175901 [222] Shaughnessy D A, Velsko C A, Jedlovec D R, Yeamans C B, [236] Frenje J A et al 2016 The magnetic recoil spectrometer Moody K J, Tereshatov E, Stoeffl W and Riddle A 2012 The (MRSt) for time-resolved measurements of the neutron radiochemical analysis of gaseous samples (RAGS) spectrum at the national ignition facility (NIF) Rev. Sci. apparatus for nuclear diagnostics at the national ignition Instrum. 87 11D806 facility Rev. Sci. Instrum. 83 10D917 [237] Hilsabeck T J, Frenje J A, Hares J D and Wink C W 2016 A [223] Gostic J M, Shaughnessy D A, Moore K T, Hutcheon I D, stretch/compress scheme for a high temporal resolution Grant P M and Moody K J 2012 Solid debris collection for detector for the magnetic recoil spectrometer time (MRSt) radiochemical diagnostics at the national ignition facility Rev. Sci. Instrum. 87 11D807 Rev. Sci. Instrum. 83 10D904 [238] Parker C E et al 2018 Implementation of the foil-on-hohlraum [224] Shaughnessy D A et al 2014 Radiochemical determination of technique for the magnetic recoil spectrometer for time- Inertial Confinement Fusion capsule compression at the resolved neutron measurements at the national ignition national ignition facility Rev. Sci. Instrum. 85 063508 facility Rev. Sci. Instrum. 89 113508 [225] Stoyer M A, Velsko C A, Spears B K, Hicks D G, [239] Wink C W, Frenje J A, Hilsabeck T J, Bionta R, Khater H Y, Hudson G B, Sangster T C and Freeman C G 2012 Gatu Johnson M, Kilkenny J D, Li C K, Séguin F H and Collection of solid and gaseous samples to diagnose Inertial Petrasso R D 2016 Signal and background considerations for Confinement Fusion implosions Rev. Sci. Instrum. 83 the MRSt on the national ignition facility (NIF) Rev. Sci. 023505 Instrum. 87 11D808 [226] Galbraith J, Bettencourt R, Shaughnessy D, Gharibyan N, [240] Fatherley V E, Fittinghoff D N, Hibbard R L, Jorgenson H J, Talison B, Morris K and Smith C 2015 Vast area detection Martinez J I, Oertel J A, Schmidt D W, Waltz C S, for experimental radiochemistry (VADER) at the national Wilde C H and Volegov P L 2018 Aperture design for the ignition facility Proc. SPIE 9591 95910G third neutron and first gamma-ray imaging systems for the [227] Edwards E R, Jedlovec D R, Carrera J A and Yeamans C B national ignition facility Rev. Sci. Instrum. 89 10I127 2016 Development of enhanced, permanently-installed, [241] Dymoke-Bradshaw A K L, Hares J D, Milnes J, neutron activation diagnostic hardware for NIF J. Phys.: Herrmann H W, Horsfield C J, Gales S G, Leatherland A, Conf. Ser. 717 012091 Hilsabeck T and Kilkenny J D 2018 Development of an [228] Root J R, Jedlovec D R, Edwards E R, Yeamans C B, ultra-fast photomultiplier tube for gamma-ray Cherenkov Golod T, Hernandez J, Adams P and Brunton G 2017 detectors at the national ignition facility (PD-PMT) Rev. Sci. Development of the real time neutron activation diagnostic Instrum. 89 10I137 system for NIF Proc. SPIE 10390, Target Diagnostics [242] Geppert-Kleinrath H et al 2018 Pulse dilation gas Cherenkov Physics and Engineering for Inertial Confinement Fusion VI detector for ultra-fast gamma reaction history at the NIF Rev. 103900J Sci. Instrum. 89 10I146 [229] Schlossberg D J, Moore A S, Beeman B V, Eckart M J, [243] Kim Y et al 2012 Gamma-to-electron magnetic spectrometer Grim G P, Hartouni E P, Hatarik R, Rubery M S, Sayre D B and (GEMS): an energy-resolved γ-ray diagnostic for the Waltz C 2018 Ab initio response functions for Cherenkov based national ignition facility Rev. Sci. Instrum. 83 10D311 neutron detectors Rev. Sci. Instrum. 89 10I136 [244] Sutcliffe G et al 2018 Development and utilization of the [230] Moore A S et al 2018 A fused silica Cherenkov radiator for DT3He multi-particle backlighter for stopping-power high precision time-of-flight measurement of DT and experiments and for radiography of strong fields at the NIF neutron spectra Rev. Sci. Instrum. 89 10I120 Bull. Am. Phys. Soc., Div. Plasma Phys. 63 11

44